movenc: Handle pts == NOPTS when autoflushing

This muxer generally handles pts == NOPTS by using dts instead;
do this for consistency here as well.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -3635,7 +3635,10 @@ static int mov_write_packet(AVFormatContext *s, AVPacket *pkt)
                 // duration, but only helps for this particular track, not
                 // for the other ones that are flushed at the same time.
                 trk->track_duration = pkt->dts - trk->start_dts;
+                if (pkt->pts != AV_NOPTS_VALUE)
                     trk->end_pts = pkt->pts;
+                else
+                    trk->end_pts = pkt->dts;
                 mov_auto_flush_fragment(s, 0);
             }
         }