CLSA-2026-1773655369 gimp: Fix of 4 CVEs

CVE-2026-2044: fix uninitialized memory read in PGM file parser - CVE-2026-2045: fix heap buffer overflow in XWD file loader - CVE-2026-2048: fix out-of-bounds write in XWD file loader - CVE-2026-0797: fix missing fread return value checks in ICO file loader...