libvips: A well-crafted PPM image processed via a custom source could lead to possible heap buffer write overflow

libvips is a fast image processing library with low memory needs. Prior to version 8.18.3, applications that define unusual custom libvips sources and use them to process untrusted uncompressed PPM images can trigger a max/min error in vips_source_read_to_memory in libvips/iofuncs/source.c. The function uses VIPS_MAX instead of VIPS_MIN when selecting the remaining read size, allowing up to 4032 bytes to be written beyond the allocated heap buffer and causing memory corruption or a process crash. This issue is fixed in version 8.18.3.