lavf/id3v2: do not export empty fields.

This also avoids a memleak.

libavformat/id3v2.c

@@ -303,6 +303,8 @@ static void read_ttag(AVFormatContext *s, AVIOContext *pb, int taglen, const cha
     }
     else if (*dst)
         dict_flags |= AV_DICT_DONT_STRDUP_VAL;
+    else
+        av_freep(&dst);
 
     if (dst)
         av_dict_set(&s->metadata, key, dst, dict_flags);