mpegvideo: Don't use ff_mspel_motion() for vc1

Using ff_mspel_motion assumes that s (a MpegEncContext
poiinter) really is a Wmv2Context.

This fixes crashes in error resilience on vc1/wmv3 videos.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
