Improve memory safety and error handling in giflib_decoder_create

Changes Made

Enhanced error handling and input validation:

• Added null pointer check for input parameter buf
• Added validation for GIF dimensions to reject invalid files with non-positive width/height
• Added overflow protection checks before memory allocation to prevent integer overflow vulnerabilities
• Added std::bad_cast exception handling for safer type casting
• Added std::length_error exception handling alongside existing std::bad_alloc handling

Modernized memory management:

• Replaced raw pointers with std::unique_ptr for automatic resource management (RAII pattern)
• Eliminated manual memset call by using std::make_unique which zero-initializes
• Used nullptr instead of NULL for modern C++ compliance
• Improved resource cleanup through RAII, reducing risk of memory leaks

Benefits

• Security: Prevents potential integer overflow attacks when processing malicious GIF files
• Reliability: More robust error handling reduces crashes from invalid inputs
• Memory Safety: RAII pattern ensures proper cleanup even when exceptions occur
• Code Quality: Follows modern C++ best practices and reduces manual memory management