lavf: make compute_chapters_end less picky.

In particular, now it assumes that
a) chapters are chronologically ordered
b) chapters have the same timebases
c) duration of the stream is known
and asserts if any of these is not met.

Make it properly deal with harsher conditions.

fixes issue2320

libavformat/utils.c

@@ -2168,22 +2168,23 @@ enum CodecID av_codec_get_id(const AVCodecTag * const *tags, unsigned int tag)
 
 static void compute_chapters_end(AVFormatContext *s)
 {
-    unsigned int i;
+    unsigned int i, j;
+    int64_t max_time = s->duration + (s->start_time == AV_NOPTS_VALUE) ? 0 : s->start_time;
 
-    for (i=0; i+1<s->nb_chapters; i++)
+    for (i = 0; i < s->nb_chapters; i++)
         if (s->chapters[i]->end == AV_NOPTS_VALUE) {
-            assert(s->chapters[i]->start <= s->chapters[i+1]->start);
-            assert(!av_cmp_q(s->chapters[i]->time_base, s->chapters[i+1]->time_base));
-            s->chapters[i]->end = s->chapters[i+1]->start;
+            AVChapter *ch = s->chapters[i];
+            int64_t   end = max_time ? av_rescale_q(max_time, AV_TIME_BASE_Q, ch->time_base)
+                                     : INT64_MAX;
+
+            for (j = 0; j < s->nb_chapters; j++) {
+                AVChapter *ch1 = s->chapters[j];
+                int64_t next_start = av_rescale_q(ch1->start, ch1->time_base, ch->time_base);
+                if (j != i && next_start > ch->start && next_start < end)
+                    end = next_start;
+            }
+            ch->end = (end == INT64_MAX) ? ch->start : end;
         }
-
-    if (s->nb_chapters && s->chapters[i]->end == AV_NOPTS_VALUE) {
-        assert(s->start_time != AV_NOPTS_VALUE);
-        assert(s->duration > 0);
-        s->chapters[i]->end = av_rescale_q(s->start_time + s->duration,
-                                           AV_TIME_BASE_Q,
-                                           s->chapters[i]->time_base);
-    }
 }
 
 static int get_std_framerate(int i){