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.

This fixes the issues I was having with rtsp seeks in the testsuite.

It might make more sense to be able to update the basesrc->segment_seqnum since we aren't using the basesrc's seek function. Not sure if we can just save the event seqnum from the FLUSH_STOP event, or need to add a new function. Then we wouldn't have to remember the seqnum in rtspsrc.