fix/close-invoke-race: fixing logical race when Close is called befor…

…e Invoke

client.go

@@ -335,6 +335,11 @@ 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 {
+	if cc.conn == nil {
+		// Close() has been called
+		return errors.New("client Close() called")
+	}
+
 	// Ensure the connection state is ready
 	cc.mu.RLock()
 	cc.conn.mu.RLock()