Do not update _top before resizing _stack

[unknown-1-0]

When SQVM::EnterFrame is called, it updates the value of _top BEFORE checking if that new value is within _stack. When this function encounters a stack overflow while in a metamethod, it returns without reverting the previous value of _top. This leads to _top still pointing outside the bounds of _stack, which later leads to out-of-bounds access and heap corruption.
The reproducer code is provided below:

import sys

if len(sys.argv) < 3:
    print("Usage: ./gen_locals_stack_overflow_repro.py <number of locals inside _get> <number of locals outside _get>")
    exit(1)

locals_inside_get = int(sys.argv[1])
locals_outside_get = int(sys.argv[2])

print(f"Locals inside _get: {locals_inside_get}")
print(f"Locals outside _get: {locals_outside_get}")


out_path="locals_stack_overflow.nut"
f=open(out_path,"w")

f.write("""local a = {
    calls = 0
    function _get(x) {
        calls += 1;
        print("Call #" + calls + ", x = " + x + "\\n");
        nonexistent_var_2; // triggers _get(x) with x = nonexistent_var_2

""")

for i in range(locals_inside_get):
    f.write(" "*4*2 + f"local _{i:02x} = {hex(i)}\n")

f.write(" "*4 + "}" + """
}
""")

for i in range(locals_outside_get):
    f.write(f"local _{i:02x} = {hex(i)}\n")

f.write("""

local b = setdelegate(a)
local b = nonexistent_var // triggers _get(x) with x = nonexistent_var
""")

f.close()

print(f"Repro saved to {out_path}")

$ python ./gen_locals_stack_overflow_repro.py 254 0
Locals inside _get: 254
Locals outside _get: 0
Repro saved to locals_stack_overflow.nut
$ mkdir build-asan
$ cd build-asan
$ CFLAGS='-fsanitize=address,undefined' CXXFLAGS='-fsanitize=address,undefined' cmake .. -DCMAKE_BUILD_TYPE=Debug
<cmake output omitted for brevity>
$ make
<make output omitted for brevity>
$ ./bin/sq ../locals_stack_overflow.nut # assuming the python script was run in the root of the repo
Call #1, x = nonexistent_var
Call #2, x = nonexistent_var_2
Call #3, x = nonexistent_var_2
=================================================================
==<sq PID here>==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7de54d5e4270 at pc 0x7f554f42a664 bp 0x7fff835f4620 sp 0x7fff835f4618
READ of size 4 at 0x7de54d5e4270 thread T0                                                                                                                                      
    #0 0x7f554f42a663 in SQObjectPtr::Null() <...>/squirrel/squirrel/sqobject.h:289
    #1 0x7f554f4c0f15 in SQVM::Pop(long long) <...>/squirrel/squirrel/sqvm.cpp:1761
    #2 0x7f554f4e5100 in SQVM::FallBackGet(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&) <...>/squirrel/squirrel/sqvm.cpp:1366
    #3 0x7f554f4e6049 in SQVM::Get(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&, unsigned long long, long long) <...>/squirrel/squirrel/sqvm.cpp:1299
    #4 0x7f554f4cbc87 in SQVM::Execute(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long, SQVM::ExecutionType) <...>/squirrel/squirrel/sqvm.cpp:853
    #5 0x7f554f4e06e6 in SQVM::Call(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long) <...>/squirrel/squirrel/sqvm.cpp:1610
    #6 0x7f554f4e50d8 in SQVM::FallBackGet(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&) <...>/squirrel/squirrel/sqvm.cpp:1361
    #7 0x7f554f4e6049 in SQVM::Get(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&, unsigned long long, long long) <...>/squirrel/squirrel/sqvm.cpp:1299
    #8 0x7f554f4cbc87 in SQVM::Execute(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long, SQVM::ExecutionType) <...>/squirrel/squirrel/sqvm.cpp:853
    #9 0x7f554f4e06e6 in SQVM::Call(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long) <...>/squirrel/squirrel/sqvm.cpp:1610
    #10 0x7f554f4e50d8 in SQVM::FallBackGet(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&) <...>/squirrel/squirrel/sqvm.cpp:1361
    #11 0x7f554f4e6049 in SQVM::Get(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&, unsigned long long, long long) <...>/squirrel/squirrel/sqvm.cpp:1299
    #12 0x7f554f4cbc87 in SQVM::Execute(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long, SQVM::ExecutionType) <...>/squirrel/squirrel/sqvm.cpp:853
    #13 0x7f554f4e06e6 in SQVM::Call(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long) <...>/squirrel/squirrel/sqvm.cpp:1610
    #14 0x7f554f4e50d8 in SQVM::FallBackGet(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&) <...>/squirrel/squirrel/sqvm.cpp:1361
    #15 0x7f554f4e6049 in SQVM::Get(SQObjectPtr const&, SQObjectPtr const&, SQObjectPtr&, unsigned long long, long long) <...>/squirrel/squirrel/sqvm.cpp:1299
    #16 0x7f554f4cbc87 in SQVM::Execute(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long, SQVM::ExecutionType) <...>/squirrel/squirrel/sqvm.cpp:853
    #17 0x7f554f4e06e6 in SQVM::Call(SQObjectPtr&, long long, long long, SQObjectPtr&, unsigned long long) <...>/squirrel/squirrel/sqvm.cpp:1610
    #18 0x7f554f41bc05 in sq_call <...>/squirrel/squirrel/sqapi.cpp:1178
    #19 0x55d51ea55245 in getargs <...>/squirrel/sq/sq.c:193
    #20 0x55d51ea56137 in main <...>/squirrel/sq/sq.c:330
    #21 0x7f554e629f67 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #22 0x7f554e62a024 in __libc_start_main_impl ../csu/libc-start.c:360
    #23 0x55d51ea54420 in _start (<...>/squirrel/build-asan-gcc/bin/sq+0x4420) (BuildId: 3d33fe8dc07ff2144c2c0957793b58cb63a6d328)

0x7de54d5e4270 is located 112 bytes after 16384-byte region [0x7de54d5e0200,0x7de54d5e4200)
allocated by thread T0 here:                                                                                                                                                    
    #0 0x7f554f918fcb in realloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:81
    #1 0x7f554f4950a1 in sq_vm_realloc(void*, unsigned long long, unsigned long long) <...>/squirrel/squirrel/sqmem.cpp:8
    #2 0x7f554f42ee73 in sqvector<SQObjectPtr>::_realloc(unsigned long long) <...>/squirrel/squirrel/squtils.h:110
    #3 0x7f554f42f00f in sqvector<SQObjectPtr>::resize(unsigned long long, SQObjectPtr const&) <...>/squirrel/squirrel/squtils.h:58
    #4 0x7f554f4c34c1 in SQVM::Init(SQVM*, long long) <...>/squirrel/squirrel/sqvm.cpp:357
    #5 0x7f554f40eaba in sq_open <...>/squirrel/squirrel/sqapi.cpp:51
    #6 0x55d51ea560d3 in main <...>/squirrel/sq/sq.c:314
    #7 0x7f554e629f67 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58

SUMMARY: AddressSanitizer: heap-buffer-overflow <...>/squirrel/squirrel/sqobject.h:289 in SQObjectPtr::Null()
Shadow bytes around the buggy address:
  0x7de54d5e3f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7de54d5e4000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7de54d5e4080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7de54d5e4100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7de54d5e4180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x7de54d5e4200: fa fa fa fa fa fa fa fa fa fa fa fa fa fa[fa]fa
  0x7de54d5e4280: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x7de54d5e4300: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x7de54d5e4380: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x7de54d5e4400: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x7de54d5e4480: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==<sq PID here>==ABORTING

This pull request fixes this bug by updating the value of _top AFTER the new top value is checked and _stack is resized (if needed).
Below is the output of the same reproducer run on squirrel with the patch applied and ASAN enabled:

$ cd build-asan
$ make
<make output omitted for brevity>
$ ./bin/sq ../locals_stack_overflow.nut
Call #1, x = nonexistent_var
Call #2, x = nonexistent_var_2
Call #3, x = nonexistent_var_2

AN ERROR HAS OCCURRED [stack overflow, cannot resize stack while in a metamethod]

CALLSTACK
*FUNCTION [main()] ../locals_stack_overflow.nut line [267]

LOCALS
[b] TABLE
[a] TABLE
[vargv] ARRAY
[this] TABLE
$