ffplay h263: crash with lowres while seeking (regression)

[ami_stuff]

to reproduce please run:

ffplay -lowres 3 QT-H263.mov

and seek a few times with the mouse

index 9ab52ef..6893a3a 100644

--- a/libavcodec/mpegvideo.c
+++ b/libavcodec/mpegvideo.c
@@ -1134,7 +1134,7 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             if(ff_alloc_picture(s, s->last_picture_ptr, 0) < 0)
                 return -1;
 
-            if(s->codec_id == CODEC_ID_FLV1){
+            if(s->codec_id == CODEC_ID_FLV1 || s->codec_id == CODEC_ID_H263){
                 for(i=0; i<s->height; i++)
                     memset(s->last_picture_ptr->f.data[0] + s->last_picture_ptr->f.linesize[0]*i, 16, s->width);
             }