id3v2: don't overwrite existing tags

Apparently some broken taggers prepend a new ID3v2 tag leaving the
existing one intact. Our parser currently reads all tags and overwrites
existing values with supposedly outdated ones.

fixes issue2419
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 75aded83)

libavformat/id3v2.c

@@ -132,7 +132,7 @@ static void read_ttag(AVFormatContext *s, ByteIOContext *pb, int taglen, const c
         val = dst;
 
     if (val)
-        av_metadata_set2(&s->metadata, key, val, 0);
+        av_metadata_set2(&s->metadata, key, val, AV_METADATA_DONT_OVERWRITE);
 }
 
 static void ff_id3v2_parse(AVFormatContext *s, int len, uint8_t version, uint8_t flags)