fix(cgif_rgb): add NULL checks after allocations

[uwezkhan]

fix(cgif_rgb): add NULL checks after allocations

Add missing NULL pointer checks after malloc/realloc calls to prevent
crashes on memory allocation failures. This fixes critical security
vulnerabilities where NULL pointers were dereferenced without checking.

Functions modified:

• init_col_hash_table: Check malloc return value
• resize_col_hash_table: Check realloc and malloc return values
• hash_to_dense: Check malloc return value
• new_tree_node: Check malloc return value
• quantize_and_dither: Check malloc, cleanup decision tree on failure
• cgif_rgb_newgif: Check malloc return value
• cgif_rgb_addframe: Check malloc twice, cleanup on failure

Impact:

• Prevents guaranteed crashes in low-memory conditions
• Properly propagates CGIF_EALLOC error code
• Adds cleanup for partially allocated memory
• No changes to success path or API

Fixes: CWE-476 (NULL Pointer Dereference)
Total: 7 functions fixed, 31 lines added

[uwezkhan]

Hey i have added a PR please check