[Marti2203]

This fixes undefined behavior in yaml_string_extend, yaml_stack_extend, and yaml_queue_extend.

After a successful realloc, pointers into the original allocation are invalid. The functions previously calculated sizes and offsets using those invalidated pointers after calling yaml_realloc.

The change calculates all required sizes and offsets before reallocating, then reconstructs the internal pointers relative to the new allocation.

Reproduction

The issue is detected with Clang’s pointer-pair sanitizer:

-fsanitize=address,pointer-subtract
ASAN_OPTIONS=detect_invalid_pointer_pairs=2:halt_on_error=1

Before the fix, extending a string reports:

  ERROR: AddressSanitizer: invalid-pointer-pair
      yaml_string_extend src/api.c

The same pattern existed in the stack and queue extension helpers.

Tests

Added test-realloc, which exercises:

• String extension while preserving the current pointer offset.
• Stack extension while preserving the top offset.
• Queue extension while preserving head and tail offsets.

Verified with:

• CMake test suite: 4/4 passed.
• Autotools test suite: 4/4 passed.
• ASan with pointer-pair detection enabled.
• A public parser test using a long scalar that forces string reallocation.