Update status on CVE-2018-6392/ffmpeg

The vulnerable code, the out of array access in the filter_slice
function is present at least in the version in unstable (unless
something done wrong during triage), and should be present as well in
the streth version:

https://sources.debian.org/src/ffmpeg/7:3.2.9-1%7Edeb9u1/libavfilter/vf_transpose.c/#L151

Upstream has adressed the out of array access in

https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/c6939f65a116b1ffed345d29d8621ee4ffb32235

but that needed a (functional) regression fix some days later with

https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/3f621455d62e46745453568d915badd5b1e5bcd5

This still would need an additional reviev.