UBUNTU-CVE-2026-125261: Medium severity ubuntu/ffmpeg vulnerability

FFmpeg through 9.0.2 contains an out-of-bounds read vulnerability in the SGI encoder's encodeframe() in libavcodec/sgienc.c, where unsigned size variables mis-handle negative linesize frames. Attackers can supply a video whose display matrix triggers autorotate vflip, so transcoding to SGI images reads pixel data far outside the frame and crashes the process.