workaround dropable p frame after first frame bug

Originally committed as revision 1424 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/svq1.c

@@ -1093,6 +1093,10 @@ static int svq1_decode_frame(AVCodecContext *avctx,
     return result;
   }
   
+  //FIXME this avoids some confusion for "B frames" without 2 references
+  //this should be removed after libavcodec can handle more flaxible picture types & ordering
+  if(s->pict_type==B_TYPE && s->last_picture.data[0]==NULL) return buf_size;
+  
   if(avctx->hurry_up && s->pict_type==B_TYPE) return buf_size;
 
   if(MPV_frame_start(s, avctx) < 0)