CLSA-2026-1779129500 libpng15: Fix of CVE-2026-34757

CVE-2026-34757: Use snapshot-before-free and defer-free patterns to prevent use-after-free when a caller passes a pointer obtained from pnggetPLTE, pnggettRNS, pnggethIST, pnggettext, pnggetsPLT, or pnggetunknownchunks back into the corresponding setter issues 836 and 837...