avformat: only fill-in interpolated timestamps if duration is non-zero

This avoids returning duplicate timestamps for multiple packets when the
demuxer does not provide all timestamps and packet duration is not known.

libavformat/utils.c

@@ -985,6 +985,8 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
                 }
             }
 
+            if (pkt->pts != AV_NOPTS_VALUE || pkt->dts != AV_NOPTS_VALUE ||
+                duration) {
             /* presentation is not delayed : PTS and DTS are the same */
             if(pkt->pts == AV_NOPTS_VALUE)
                 pkt->pts = pkt->dts;
@@ -996,6 +998,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
                 st->cur_dts = pkt->pts + duration;
             }
         }
+    }
 
     if(pkt->pts != AV_NOPTS_VALUE && delay <= MAX_REORDER_DELAY){
         st->pts_buffer[0]= pkt->pts;