Fix parser not to clobber has_b_frames when extradata is set.

Due to it in contrast to the decoder not setting up 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.
estimate_timings_from_pts would cause the parser to be reopened
on the same stream.

This fixes trac issue #360.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/h264_parser.c

@@ -251,6 +251,12 @@ static int h264_parse(AVCodecParserContext *s,
         h->got_first = 1;
         if (avctx->extradata_size) {
             h->s.avctx = avctx;
+            // must be done like in decoder, otherwise opening the parser,
+            // letting it create extradata and then closing and opening again
+            // will cause has_b_frames to be always set.
+            // Note that estimate_timings_from_pts does exactly this.
+            if (!avctx->has_b_frames)
+                h->s.low_delay = 1;
             ff_h264_decode_extradata(h);
         }
     }