avformat/matroskadec: Use rounded down duration in get_cue_desc() check

Floating point is evil, it would be better if duration was not a double

Fixes: Infinite loop
Fixes: 45123/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-6725052291219456

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -4013,7 +4013,8 @@ static CueDesc get_cue_desc(AVFormatContext *s, int64_t ts, int64_t cues_start)
     CueDesc cue_desc;
     int i;
 
-    if (ts >= matroska->duration * matroska->time_scale) return (CueDesc) {-1, -1, -1, -1};
+    if (ts >= (int64_t)(matroska->duration * matroska->time_scale))
+        return (CueDesc) {-1, -1, -1, -1};
     for (i = 1; i < nb_index_entries; i++) {
         if (index_entries[i - 1].timestamp * matroska->time_scale <= ts &&
             index_entries[i].timestamp * matroska->time_scale > ts) {