fixing logical race when Close is called before Invoke

client.go

@@ -369,8 +369,12 @@ func (cc *ClientConn) Close() {
 // Invoke sends the RPC request on the wire and returns after response is
 // received.
 func (cc *ClientConn) Invoke(ctx context.Context, method string, args interface{}, reply interface{}) error {
-	// Ensure the connection state is ready
+	if cc.addrConn == nil {
+		// Close() has been called
+		return errors.New("client Close() called")
+	}
 
+	// Ensure the connection state is ready
 	cc.mu.RLock()
 	cc.addrConn.mu.RLock()
 	state := cc.addrConn.state