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>
(cherry picked from commit 18f2d5cb9c)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit da0c457663)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 899d95efe1)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/mpegvideo_common.h

@@ -727,7 +727,8 @@ static av_always_inline void MPV_motion_internal(MpegEncContext *s,
                         0, 0, 0,
                         ref_picture, pix_op, qpix_op,
                         s->mv[dir][0][0], s->mv[dir][0][1], 16);
-        }else if(!is_mpeg12 && (CONFIG_WMV2_DECODER || CONFIG_WMV2_ENCODER) && s->mspel){
+        } else if (!is_mpeg12 && (CONFIG_WMV2_DECODER || CONFIG_WMV2_ENCODER) &&
+                   s->mspel && s->codec_id == CODEC_ID_WMV2) {
             ff_mspel_motion(s, dest_y, dest_cb, dest_cr,
                         ref_picture, pix_op,
                         s->mv[dir][0][0], s->mv[dir][0][1], 16);