avformat/mov: Check element count in mov_metadata_hmmt()

Fixes: Timeout
Fixes: 30325/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-6048395703746560

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

@@ -288,6 +288,8 @@ static int mov_metadata_hmmt(MOVContext *c, AVIOContext *pb, unsigned len)
         return 0;
 
     n_hmmt = avio_rb32(pb);
+    if (n_hmmt > len / 4)
+        return AVERROR_INVALIDDATA;
     for (i = 0; i < n_hmmt && !pb->eof_reached; i++) {
         int moment_time = avio_rb32(pb);
         avpriv_new_chapter(c->fc, i, av_make_q(1, 1000), moment_time, AV_NOPTS_VALUE, NULL);