ffmpeg seek callback: more sanity checks
ffmpeg seek callback: more sanity checks

This also includes a check against an FFmpeg bug:
https://trac.ffmpeg.org/ticket/4038

albertz/music-player-core,etienne-gauvin/music-player-core,albertz/music-player-core,etienne-gauvin/music-player-core,albertz/music-player-core,etienne-gauvin/music-player-core