It does not make sense (DV is interleaved by design) and
it avoids a crash when the non-interleaved code tries to
use the priv_data of streams created by the DV demuxer.
The crash could be avoided differently, but then that stream
would still lack an index and would not play correctly in
non-interleaved mode.
Fixes e.g. samples/ffmpeg-bugs/roundup/issue1514/Dennis0002_video1.avi