libheif allows Out-of-bounds vector access leading to invalid dereference (DoS)

libheif is a HEIF and AVIF file format decoder and encoder. In versions 1.21.2 and prior, a malformed HEIF sequence file can trigger an out-of-bounds read in core sequence parsing logic, causing DoS. A malformed file can have stco.entry_count == 0 (creating no chunks) while still passing validation because saio.entry_count == 0 matches, but with saiz.sample_count > 0 the SampleAuxInfoReader constructor still enters its loop. This leads to an out-of-bounds dereference on the empty chunks[0] in chunked mode.