avformat/utils: keep chapter monotonicity on chapter updates

Updating a chapter with the same id does not break monotonicity
Fixes: Timeout
Fixes: 43727/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-4960623367159808

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

libavformat/utils.c

@@ -895,10 +895,11 @@ AVChapter *avpriv_new_chapter(AVFormatContext *s, int64_t id, AVRational time_ba
     if (!s->nb_chapters) {
         si->chapter_ids_monotonic = 1;
     } else if (!si->chapter_ids_monotonic || s->chapters[s->nb_chapters-1]->id >= id) {
-        si->chapter_ids_monotonic = 0;
         for (unsigned i = 0; i < s->nb_chapters; i++)
             if (s->chapters[i]->id == id)
                 chapter = s->chapters[i];
+        if (!chapter)
+            si->chapter_ids_monotonic = 0;
     }
 
     if (!chapter) {