MDEV-25472 : Server crashes when wsrep_cluster_address set to unkown address...

MDEV-25472 : Server crashes when wsrep_cluster_address set to unkown address and wsrep_slave_threads to 0

Return failure if we are not connected when slave threads are set

sql/wsrep_thd.cc

@@ -587,9 +587,8 @@ bool wsrep_create_appliers(long threads, bool thread_count_lock)
     {
       WSREP_ERROR("Trying to launch slave threads before creating "
                   "connection at '%s'", wsrep_cluster_address);
-      assert(0);
     }
-    return false;
+    return true;
   }
 
   long wsrep_threads= 0;