Dont drop B frames without last_picture.

Fixes issue1722

Originally committed as revision 21783 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -1715,11 +1715,6 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
     h->slice_type_nos= slice_type & 3;
 
     s->pict_type= h->slice_type; // to make a few old functions happy, it's wrong though
-    if (s->pict_type == FF_B_TYPE && s0->last_picture_ptr == NULL) {
-        av_log(h->s.avctx, AV_LOG_ERROR,
-               "B picture before any references, skipping\n");
-        return -1;
-    }
 
     pps_id= get_ue_golomb(&s->gb);
     if(pps_id>=MAX_PPS_COUNT){