H264 allows B frames without requiring a >=1 sized buffer.

Patch by wallak, wallak free fr

Originally committed as revision 20160 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -770,7 +770,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
     int num, den, presentation_delayed, delay, i;
     int64_t offset;
 
-    if (pc && pc->pict_type == FF_B_TYPE)
+    if (st->codec->codec_id != CODEC_ID_H264 && pc && pc->pict_type == FF_B_TYPE)
         st->codec->has_b_frames = 1;
 
     /* do we have a video B-frame ? */