Detecting missing tfhd avoids re-using tfhd track info from the previous
moof. For files with multiple tracks, this may make a mess of the
avindex and fragindex, which can later trigger av_assert0 in
mov_read_trun().
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>