parse stream headers for audio streams in mkv, needed for frame size

Originally committed as revision 22925 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1394,6 +1394,7 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
             st->codec->codec_type = AVMEDIA_TYPE_AUDIO;
             st->codec->sample_rate = track->audio.out_samplerate;
             st->codec->channels = track->audio.channels;
+            st->need_parsing = AVSTREAM_PARSE_HEADERS;
         } else if (track->type == MATROSKA_TRACK_TYPE_SUBTITLE) {
             st->codec->codec_type = AVMEDIA_TYPE_SUBTITLE;
         }