Fix bmp alloc dos

[Korolikc]

Summary

This PR fixes a denial-of-service issue in bmp_img_alloc() where crafted BMP headers
can trigger excessive memory allocations leading to OOM/abort.

Root cause

• bmp_img_alloc() performed allocations based on untrusted biWidth / biHeight
  without validating ranges or guarding against overflow.
• Allocation failures were not propagated to callers.

Changes

• Change bmp_img_alloc() to return enum bmp_error and validate dimensions:
  • Reject non-positive dimensions and INT_MIN height (abs overflow case)
  • Add SIZE_MAX overflow checks before allocation
  • Handle allocation failures and free partial allocations
• bmp_img_read() now checks allocation result and exits gracefully on failure.
• Remove meaningless abs() usage on unsigned indices.
• Minor header indentation cleanup.

Verification

A minimized PoC BMP file previously triggered ASan OOM in bmp_img_alloc().
After this patch, the same PoC is rejected and the program exits normally
(no crash / abort).