Ensure that RoundRobin.Close() does not panic. (#1139)

balancer.go

@@ -385,6 +385,9 @@ func (rr *roundRobin) Notify() <-chan []Address {
 func (rr *roundRobin) Close() error {
 	rr.mu.Lock()
 	defer rr.mu.Unlock()
+	if rr.done {
+		return errBalancerClosed
+	}
 	rr.done = true
 	if rr.w != nil {
 		rr.w.Close()

clientconn.go

@@ -78,6 +78,8 @@ var (
 	errConnClosing = errors.New("grpc: the connection is closing")
 	// errConnUnavailable indicates that the connection is unavailable.
 	errConnUnavailable = errors.New("grpc: the connection is unavailable")
+	// errBalancerClosed indicates that the balancer is closed.
+	errBalancerClosed = errors.New("grpc: balancer is closed")
 	// minimum time to give a connection to complete
 	minConnectTimeout = 20 * time.Second
 )