MDEV-23856 fts_optimize_wq accessed after shutdown of FTS Optimize thread

In fts_optimize_remove_table(), InnoDB tries to access the
fts_optimize_wq after shutting down the fts optimize thread.
This issue caused by the commit a41d4297.
Fix should check for fts optimize thread shutdown state
before checking fts_optimize_wq.

storage/innobase/fts/fts0opt.cc

@@ -2603,9 +2603,13 @@ fts_optimize_remove_table(
 
 	os_event_destroy(event);
 
-	ut_d(mutex_enter(&fts_optimize_wq->mutex));
-	ut_ad(!table->fts->in_queue);
-	ut_d(mutex_exit(&fts_optimize_wq->mutex));
+#ifdef UNIV_DEBUG
+	if (!fts_opt_start_shutdown) {
+		mutex_enter(&fts_optimize_wq->mutex);
+		ut_ad(!table->fts->in_queue);
+		mutex_exit(&fts_optimize_wq->mutex);
+	}
+#endif /* UNIV_DEBUG */
 }
 
 /** Send sync fts cache for the table.