avformat/mov: Disallow FTYP after streams

Fixes: Assertion !c->fc->nb_streams failed at libavformat/mov.c:7799
Fixes: 63875/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-5479178702815232

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -1222,6 +1222,8 @@ static int mov_read_ftyp(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     int ret = ffio_read_size(pb, type, 4);
     if (ret < 0)
         return ret;
+    if (c->fc->nb_streams)
+        return AVERROR_INVALIDDATA;
 
     if (strcmp(type, "qt  "))
         c->isom = 1;