avformat/mp3dec: optimize mp3_seek() for dir < 0

this minimizes the amount of protocol seeks and reading needed in that case
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 171dd675)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mp3dec.c

@@ -318,6 +318,8 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
         return -1;
     }
 
+    if (dir < 0)
+        avio_seek(s->pb, FFMAX(ie->pos - 4096, 0), SEEK_SET);
     ret = avio_seek(s->pb, ie->pos, SEEK_SET);
     if (ret < 0)
         return ret;