libvips: Possible heap-based buffer read overflow when resizing and re-encoding a JPEG with gain map

libvips is a fast image processing library with low memory needs. Prior to version 8.18.3, libvips built with libultrahdr support can incorrectly size an output buffer in libvips/foreign/uhdrsave.c within vips_foreign_save_uhdr_set_raw_hdr when a pipeline enlarges an incoming JPEG to a very large output before encoding a gain map through VipsForeignSaveUhdr. The undersized allocation can cause a heap buffer over-read that may disclose adjacent data or crash the process. This issue is fixed in version 8.18.3.