CVE-2026-91186 PNG decoder: a duplicate IHDR chunk after IDAT resizes the image after allocation, causing a heap out-of-bounds write

The PNG chunk reader has no notion of chunk ordering. It records whether an IDAT has been seen and whether a tRNS has already supplied transparency, but nothing prevents an IHDR being read a second time. A PNG carries exactly one IHDR and it is the first chunk. A file that places a second one aft...