avformat/utils: better probing for duration in estimate_timings_from_pts()

It seems it is more secure to simply duplicate the computing routine from compute_pkt_fields to estimate_timings_from_pts.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2477,7 +2477,7 @@ static void estimate_timings_from_pts(AVFormatContext *ic, int64_t old_offset)
 {
     AVPacket pkt1, *pkt = &pkt1;
     AVStream *st;
-    int read_size, i, ret;
+    int num, den, read_size, i, ret;
     int found_duration = 0;
     int is_end;
     int64_t filesize, offset, duration;
@@ -2525,6 +2525,15 @@ static void estimate_timings_from_pts(AVFormatContext *ic, int64_t old_offset)
             if (pkt->pts != AV_NOPTS_VALUE &&
                 (st->start_time != AV_NOPTS_VALUE ||
                  st->first_dts  != AV_NOPTS_VALUE)) {
+                if (pkt->duration == 0) {
+                    ff_compute_frame_duration(&num, &den, st, st->parser, pkt);
+                    if (den && num) {
+                        pkt->duration = av_rescale_rnd(1,
+                                           num * (int64_t) st->time_base.den,
+                                           den * (int64_t) st->time_base.num,
+                                           AV_ROUND_DOWN);
+                    }
+                }
                 duration = pkt->pts + pkt->duration;
                 found_duration = 1;
                 if (st->start_time != AV_NOPTS_VALUE)