avformat/mov: Check STCO location

Fixes: bypassing of checks and assertion failure
Fixes: asan_1003879.mp4

Found-by: Clusterfuzz + asan
Reported-by: Thomas Guilbert <tguilbert@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1cd4184020)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -1979,6 +1979,10 @@ static int mov_read_stco(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     MOVStreamContext *sc;
     unsigned int i, entries;
 
+    if (c->trak_index < 0) {
+        av_log(c->fc, AV_LOG_WARNING, "STCO outside TRAK\n");
+        return 0;
+    }
     if (c->fc->nb_streams < 1)
         return 0;
     st = c->fc->streams[c->fc->nb_streams-1];