Fix importing of AVIs without an index, broken since ffmpeg r7842
Fix importing of AVIs without an index, broken since ffmpeg r7842

JanX2/Perian,MaddTheSane/perian,MaddTheSane/perian,JanX2/Perian,MaddTheSane/perian,JanX2/Perian,JanX2/Perian,MaddTheSane/perian,MaddTheSane/perian,JanX2/Perian,MaddTheSane/perian