fix: #292 | EOF and 0-byte infinite loop

[winex]

fix #292 | tldr; "infinite loop in interactive interpreter on EOF and 0-byte"
thanks @Keith-S-Thompson in #292 (comment) for pointing this out and providing a patch, but '\0' should also be checked and doing this last

my tests pass (insignificant lines removed):

• Ctrl-D (^D) , EOF
• '\n'
• '\0'+
• $ cat /dev/random | squirrel3...

$  ./squirrel3
sq> ^D
$  echo | ./squirrel3
sq>
sq>
$  echo -n | ./squirrel3
sq>
$  cat /dev/null | ./squirrel3
sq>
$  cat /dev/zero | ./squirrel3
sq>

note: echoing newline looks correct - 2 prompts

well, /dev/random doesn't seem to be correct input ;p , so i haven't checked that thoroughly:

$  cat /dev/random | ./squirrel3
sq>
interactive console line = (1) column = (0) : error unexpected character(control)
$  cat /dev/random | ./squirrel3
sq>
interactive console line = (1) column = (1) : error expression expected

at least it seem to stop.

let me know if anything more should be checked/tested. this loop still looks like bad to me...