Make h264 decoder conform to requirements of mpegvideo's MPV_frame_start.

Namely, that it should not be called if you are starting to decode a B
frame without any reference pictures.
Prevents an endless allocation cycle in MPV_frame_start that will end in
picture buffer overflow and abort.
Fixes roundup issue 216.

Originally committed as revision 11214 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3911,6 +3911,11 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
     h->slice_type= slice_type;
 
     s->pict_type= h->slice_type; // to make a few old func happy, it's wrong though
+    if (s->pict_type == B_TYPE && s->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){