CLSA-2026-1778928586 libpng: Fix of CVE-2026-34757

CVE-2026-34757: use-after-free in pngsetPLTE, pngsettRNS, pngsethIST, pngsettext, pngsetsPLT and pngsetunknownchunks when the caller passes the pointer returned by the corresponding getter back to the setter...