lavf: initialize cur_dts to AV_NOPTS_VALUE for muxing

The reasoning for setting it to zero only applies to demuxing.

libavformat/utils.c

@@ -2549,16 +2549,18 @@ AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
 
         /* default pts setting is MPEG-like */
         avpriv_set_pts_info(st, 33, 1, 90000);
-    }
-
-    st->index      = s->nb_streams;
-    st->start_time = AV_NOPTS_VALUE;
-    st->duration   = AV_NOPTS_VALUE;
         /* we set the current DTS to 0 so that formats without any timestamps
          * but durations get some timestamps, formats with some unknown
          * timestamps have their first few packets buffered and the
          * timestamps corrected before they are returned to the user */
         st->cur_dts = 0;
+    } else {
+        st->cur_dts = AV_NOPTS_VALUE;
+    }
+
+    st->index      = s->nb_streams;
+    st->start_time = AV_NOPTS_VALUE;
+    st->duration   = AV_NOPTS_VALUE;
     st->first_dts     = AV_NOPTS_VALUE;
     st->probe_packets = MAX_PROBE_PACKETS;