qtdemux: restore the segment on case of soft reset

When seeking back to restore the mdat position a flush is pushed
through and it resets downstream segment information. Make sure
that after the flush (that does a soft reset) a segment will
be pushed again

Fixes regressions spotted at
https://ci.gstreamer.net/job/GStreamer-master-validate/2100/

gst/isomp4/qtdemux.c

@@ -1995,6 +1995,9 @@ gst_qtdemux_reset (GstQTDemux * qtdemux, gboolean hard)
       qtdemux->streams[n]->time_position = 0;
       qtdemux->streams[n]->accumulated_base = 0;
     }
+    if (!qtdemux->pending_newsegment) {
+      qtdemux->pending_newsegment = gst_event_new_segment (&qtdemux->segment);
+    }
   }
 }