device manager: don't do operations on nil pointer.

If grpc.DialContext() fails, a nil connection is returned. Check the
error before calling conn.Close().

pkg/kubelet/cm/devicemanager/device_plugin_stub.go

@@ -165,10 +165,10 @@ func (m *Stub) Register(kubeletEndpoint, resourceName string, pluginSockDir stri
 		grpc.WithDialer(func(addr string, timeout time.Duration) (net.Conn, error) {
 			return net.DialTimeout("unix", addr, timeout)
 		}))
-	defer conn.Close()
 	if err != nil {
 		return err
 	}
+	defer conn.Close()
 	client := pluginapi.NewRegistrationClient(conn)
 	reqt := &pluginapi.RegisterRequest{
 		Version:      pluginapi.Version,