Only read ID3v1 tag if ID3v2 isn't present or is empty.

This prevents the demuxer from exporting multiple semantically identical but
differently named tags.

Patch by Anton Khirnov, wyskas gmail

Originally committed as revision 20175 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mp3.c

@@ -142,8 +142,9 @@ static int mp3_read_header(AVFormatContext *s,
     st->need_parsing = AVSTREAM_PARSE_FULL;
     st->start_time = 0;
 
-    ff_id3v1_read(s);
     ff_id3v2_read(s);
+    if (!av_metadata_get(s->metadata, "", NULL, AV_METADATA_IGNORE_SUFFIX))
+        ff_id3v1_read(s);
 
     off = url_ftell(s->pb);
     if (mp3_parse_vbr_tags(s, st, off) < 0)