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: <gstreamer/gst-plugins-good!957>
33 jobs for !957 with backport-mr-954-into-1.18-fix_rtpspsrc_seek in 32 minutes and 45 seconds (queued for 4 seconds)
merge request