Add overflow protection and memory safety improvements to bigint operations:
- Add overflow check in mpz_realloc to prevent integer overflow in size calculations
- Fix zero-initialization loop by preserving original size during reallocation
- Improve mpz_clear to prevent double-free by nullifying pointer after free
- Add bounds checking to mpz_get_str for string conversion buffer allocation
- Add documentation comments clarifying memory allocation strategies
- Add helper macros MPZ_TMP_INIT/CLEAR for safer temporary variable management
These changes prevent potential memory corruption, buffer overflows, and crashes
while maintaining full compatibility with existing bigint functionality.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
d7ef4355a9