Improve C ABI struct change error handling (#2386)

Over in #2369 I saw this C ABI change error 

```
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.11.15/x64/bin/check-c-abi", line 6, in <module>
    sys.exit(main_cli())
using dlpack from /home/runner/work/cuvs/cuvs/dlpack/include
             ^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.11.15/x64/lib/python3.11/site-packages/check_c_abi/main.py", line 136, in main_cli
    errors = analyze_c_abi(old_abi, new_abi)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.11.15/x64/lib/python3.11/site-packages/check_c_abi/abi.py", line 351, in analyze_c_abi
    errors.extend(_analyze_struct_abi(old_abi, new_abi))
  File "/opt/hostedtoolcache/Python/3.11.15/x64/lib/python3.11/site-packages/check_c_abi/abi.py", line 293, in _analyze_struct_abi
    new_struct.members,
    ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'new_struct' where it is not associated with a value
```

This PR adds a `continue` so we avoid any unbounded variables.

Authors:
  - James Bourbeau (https://github.com/jrbourbeau)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - Gil Forsyth (https://github.com/gforsyth)

URL: #2386

Author: jrbourbeau

ci/check_c_abi/check_c_abi/abi.py

@@ -1,5 +1,5 @@
 #
-# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
 
@@ -287,6 +287,7 @@ def _analyze_struct_abi(
                 symbol=name,
                 location=old_struct.location,
             )
+            continue
 
         for (old_type, old_name), (new_type, new_name) in zip_longest(
             old_struct.members,

ci/check_c_abi/check_c_abi/tests/test_abi.py

@@ -1,5 +1,5 @@
 #
-# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
 
@@ -65,6 +65,13 @@ def test_struct():
     errors = analyze_c_abi(old_abi, old_abi)
     assert not errors
 
+    # removing a struct should return an error
+    new_abi = abi_from_str("")
+    errors = analyze_c_abi(old_abi, new_abi)
+    assert len(errors) == 1
+    assert errors[0].symbol == "Foo"
+    assert errors[0].error == "Struct has been removed"
+
     # removing a field should return an error
     new_abi = abi_from_str("""
         struct Foo {