oggvorbis: move handling of first packets ts from parser to muxer.

The parser does not have enough knowledge it seems to do it
correctly.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vorbis_parser.c

@@ -226,7 +226,6 @@ int avpriv_vorbis_parse_frame(VorbisParseContext *s, const uint8_t *buf,
             previous_blocksize = s->blocksize[flag];
         }
         current_blocksize     = s->mode_blocksize[mode];
-        if(previous_blocksize)
         duration              = (previous_blocksize + current_blocksize) >> 2;
         s->previous_blocksize = current_blocksize;
     }

libavformat/oggparsevorbis.c

@@ -323,7 +323,7 @@ static int vorbis_packet(AVFormatContext *s, int idx)
             }
             next_pkt += os->segments[seg];
         }
-        os->lastpts = os->lastdts   = os->granule - duration;
+        os->lastpts = os->lastdts   = os->granule - FFMIN(duration, os->granule);
         if(s->streams[idx]->start_time == AV_NOPTS_VALUE) {
             s->streams[idx]->start_time = os->lastpts;
             if (s->streams[idx]->duration)