avformat/utils: Don't compare pointers when sorting chapters

The chapters are independently allocated, so that comparing
the pointers is undefined behaviour. Furthermore, its result
is not platform-independent (and may not even be deterministic
on a particular platform). So compare the chapters' ids instead.
(avpriv_new_chapter() ensures that there are no duplicate ids.)

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/utils.c

@@ -3160,7 +3160,7 @@ static int chapter_start_cmp(const void *p1, const void *p2)
     int delta = av_compare_ts(ch1->start, ch1->time_base, ch2->start, ch2->time_base);
     if (delta)
         return delta;
-    return (ch1 > ch2) - (ch1 < ch2);
+    return FFDIFFSIGN(ch1->id, ch2->id);
 }
 
 static int compute_chapters_end(AVFormatContext *s)