avformat: Change avpriv_new_chapter() from O(n) to (1) in the common case

Fixes: timeout (slow -> 300ms)
Fixes: 28876/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-5664824587583488

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

libavformat/internal.h

@@ -142,6 +142,11 @@ struct AVFormatInternal {
      * Prefer the codec framerate for avg_frame_rate computation.
      */
     int prefer_codec_framerate;
+
+    /**
+     * Set if chapter ids are strictly monotonic.
+     */
+    int chapter_ids_monotonic;
 };
 
 struct AVStreamInternal {

libavformat/utils.c

@@ -4614,9 +4614,14 @@ AVChapter *avpriv_new_chapter(AVFormatContext *s, int id, AVRational time_base,
         return NULL;
     }
 
-    for (i = 0; i < s->nb_chapters; i++)
-        if (s->chapters[i]->id == id)
-            chapter = s->chapters[i];
+    if (!s->nb_chapters) {
+        s->internal->chapter_ids_monotonic = 1;
+    } else if (!s->internal->chapter_ids_monotonic || s->chapters[s->nb_chapters-1]->id >= id) {
+        s->internal->chapter_ids_monotonic = 0;
+        for (i = 0; i < s->nb_chapters; i++)
+            if (s->chapters[i]->id == id)
+                chapter = s->chapters[i];
+    }
 
     if (!chapter) {
         chapter = av_mallocz(sizeof(AVChapter));