don't try to add pool id if pool doesn't exist

pkg/cloudprovider/providers/openstack/openstack_loadbalancer.go

@@ -1144,12 +1144,14 @@ func (lbaas *LbaasV2) EnsureLoadBalancerDeleted(clusterName string, service *v1.
 		if err != nil && err != ErrNotFound {
 			return fmt.Errorf("Error getting pool for listener %s: %v", listener.ID, err)
 		}
+		if pool != nil {
 			poolIDs = append(poolIDs, pool.ID)
 			// If create-monitor of cloud-config is false, pool has not monitor.
 			if pool.MonitorID != "" {
 				monitorIDs = append(monitorIDs, pool.MonitorID)
 			}
 		}
+	}
 
 	// get all members associated with each poolIDs
 	var memberIDs []string