CImg Library: Uncontrolled memory allocation via nb_colors field in _load_bmp

CImg Library is a C++ library for image processing. Prior to commit c3aacf5, the nb_colors field read from the BMP file header is used directly to compute an allocation size without validating it against the remaining file size. A crafted BMP file with a large nb_colors value triggers an out-of-memory condition, crashing any application that uses CImg to load untrusted BMP files. This issue has been patched via commit c3aacf5.