avformat/mp3dec: Ensure the buffer is large enough to avoid seeks from the first frame search

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mp3dec.c

@@ -374,6 +374,8 @@ static int mp3_read_header(AVFormatContext *s)
 
     off = avio_tell(s->pb);
     for (i = 0; i < 64 * 1024; i++) {
+        if (!(i&1023))
+            ffio_ensure_seekback(s->pb, i + 1024 + 4);
         if (check(s, off + i) >= 0) {
             av_log(s, AV_LOG_INFO, "Skipping %d bytes of junk at %lld.\n", i, (long long)off);
             avio_seek(s->pb, off + i, SEEK_SET);