Vulnerability is in libheif's HEIF sequence parser
(Box_stsz::parse in libheif/sequences/seq_boxes.cc, plus track.cc /
chunk.cc). This sequence / ISOBMFF track parsing was added in v1.20.0;
v1.19.8 — the version we ship — has no sequences/ directory and no
Box_stsz at all (verified: no such file or symbol in the v1.19.8 source),
so the vulnerable code is not present. The GHSA lists "affected >= 1.19.0",
but that range is imprecise: the introducing commit 58c21087 is not in the
v1.19.8 tag.