frame-xchg: Allow calling frame_xchg_stop from the callback

Don't crash if the user calls frame_xchg_stop(wdev) from inside the
frame exchange's final callback.  That call is going to be redundant but
it's convenient to do this inside a cleanup function for a given wdev
without having to check whether any frame exchange was actually running.

src/frame-xchg.c

@@ -1266,9 +1266,10 @@ error:
 static void frame_xchg_exit(void)
 {
 	struct l_queue *groups = watch_groups;
+	struct l_queue *xchgs = frame_xchgs;
 
-	l_queue_destroy(frame_xchgs, frame_xchg_cancel);
 	frame_xchgs = NULL;
+	l_queue_destroy(xchgs, frame_xchg_cancel);
 
 	watch_groups = NULL;
 	l_queue_destroy(groups, frame_watch_group_destroy);