mpegvideo: remove last_picture_ptr / h264 assert.

This assert is no longer true since h264 error concealment needs
last_picture_ptr to be set.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/mpegvideo.c

@@ -1175,9 +1175,6 @@ int ff_MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
     Picture *pic;
     s->mb_skipped = 0;
 
-    assert(s->last_picture_ptr == NULL || s->out_format != FMT_H264 ||
-           s->codec_id == AV_CODEC_ID_SVQ3);
-
     /* mark & release old frames */
     if (s->out_format != FMT_H264 || s->codec_id == AV_CODEC_ID_SVQ3) {
         if (s->pict_type != AV_PICTURE_TYPE_B && s->last_picture_ptr &&