avi: properly fail if the dv demuxer is missing

CC: libav-stable@libav.org
avi: DV in AVI must be considered single stream

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
avi: directly resync on DV in AVI read failure

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org