avformat/mp3dec: Check header_filesize

Fixes: CID1608714 Division or modulo by float zero

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit cea4dbc903)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mp3dec.c

@@ -585,7 +585,7 @@ static int mp3_seek(AVFormatContext *s, int stream_index, int64_t timestamp,
     if (best_pos < 0)
         return best_pos;
 
-    if (mp3->is_cbr && ie == &ie1 && mp3->frames) {
+    if (mp3->is_cbr && ie == &ie1 && mp3->frames && mp3->header_filesize > 0) {
         int frame_duration = av_rescale(st->duration, 1, mp3->frames);
         ie1.timestamp = frame_duration * av_rescale(best_pos - si->data_offset, mp3->frames, mp3->header_filesize);
     }