Fix parser not to clobber has_b_frames when extradata is set.

Because in contrast to the decoder, the parser does not setup low_delay.
The code in parse_nal_units would always end up setting has_b_frames
to "1", except when stream is explicitly marked as low delay.
Since the parser itself would create 'extradata', simply reopening
the parser would cause this.

This happens for instance in estimate_timings_from_pts(), which causes the
parser to be reopened on the same stream.

This fixes Libav #22 and FFmpeg (trac) #360

CC: libav-stable@libav.org

Based on a patch by Reimar Döffinger <Reimar.Doeffinger@gmx.de>
(commit 31ac0ac2)

Comments and description adapted by Reinhard Tartler.
Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/h264_parser.c

@@ -251,6 +251,13 @@ static int h264_parse(AVCodecParserContext *s,
         h->got_first = 1;
         if (avctx->extradata_size) {
             h->s.avctx = avctx;
+            // must be done like in the decoder.
+            // otherwise opening the parser, creating extradata,
+            // and then closing and opening again
+            // will cause has_b_frames to be always set.
+            // NB: estimate_timings_from_pts behaves exactly like this.
+            if (!avctx->has_b_frames)
+                h->s.low_delay = 1;
             ff_h264_decode_extradata(h);
         }
     }