Assertion failure (abort/DoS) in jxl::PlaneBase::PlaneBase() reachable only when
the cjxl encoder is used to encode a maliciously crafted GIF. There is no memory
corruption and no upstream fix: the upstream tracking issues (#422, #762) remain
open with no merged commit, and Debian rates the issue "unimportant" with a
"Negligible security impact" note and no fixed version in any suite (bookworm
through sid). With no upstream remediation available and negligible impact on the
libjxl decode path we ship, this is not actionable as a backport.