rtspsrc: Fix race saving seek event seqnum.

We need to save the seek seqnum before the flush stop event
since that will start the basesrc task which may send the segment
event before we're ready.

Part-of: <!954>

gst/rtsp/gstrtspsrc.c

@@ -2875,6 +2875,11 @@ gst_rtspsrc_perform_seek (GstRTSPSrc * src, GstEvent * event)
   /* PLAY will add the range header now. */
   src->need_range = TRUE;
 
+  /* If an accurate seek was requested, we want to clip the segment we
+   * output in ONVIF mode to the requested bounds */
+  src->clip_out_segment = ! !(flags & GST_SEEK_FLAG_ACCURATE);
+  src->seek_seqnum = gst_event_get_seqnum (event);
+
   /* prepare for streaming again */
   if (flush) {
     /* if we started flush, we stop now */
@@ -2923,11 +2928,6 @@ gst_rtspsrc_perform_seek (GstRTSPSrc * src, GstEvent * event)
       seek_style = "Next";
   }
 
-  /* If an accurate seek was requested, we want to clip the segment we
-   * output in ONVIF mode to the requested bounds */
-  src->clip_out_segment = ! !(flags & GST_SEEK_FLAG_ACCURATE);
-  src->seek_seqnum = gst_event_get_seqnum (event);
-
   if (playing)
     gst_rtspsrc_play (src, &seeksegment, FALSE, seek_style);