tsdemux: Reset pending_ts on a discont, to re-measure initial timestamps

Since all the other timestamp tracking now gets reset on a discont,
it makes sense to wait for a PCR and timestamp buffers like when
playback first starts

gst/mpegtsdemux/tsdemux.c

@@ -1229,6 +1229,7 @@ gst_ts_demux_stream_flush (TSDemuxStream * stream, GstTSDemux * tsdemux)
   stream->first_dts = GST_CLOCK_TIME_NONE;
   stream->raw_pts = -1;
   stream->raw_dts = -1;
+  stream->pending_ts = TRUE;
   stream->continuity_counter = CONTINUITY_UNSET;
 }