avformat/mov: Do not use reference stream in mov_read_sidx() if there is no reference stream

Fixes: NULL pointer dereference
Fixes: clusterfuzz-testcase-minimized-audio_decoder_fuzzer-5634316373721088

Reported-by: Chris Cunningham <chcunningham@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b0d8b7cb8e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -5050,7 +5050,7 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 }
             }
         }
-        for (i = 0; i < c->fc->nb_streams; i++) {
+        if (ref_st) for (i = 0; i < c->fc->nb_streams; i++) {
             st = c->fc->streams[i];
             sc = st->priv_data;
             if (!sc->has_sidx) {