Fix the crash caused by interface null value judgment error in golang. (alibaba#2245)

* fix init docker client

* update test

Author: linrunqi08

pkg/helper/containercenter/container_center.go

@@ -1188,6 +1188,7 @@ func (dc *ContainerCenter) initClient() error {
 	// do not CreateDockerClient multi times
 	if dc.client == nil {
 		if dc.client, err = CreateDockerClient(); err != nil {
+			dc.client = nil
 			dc.setLastError(err, "init docker client from env error")
 			return err
 		}

pkg/helper/containercenter/container_center_test.go

@@ -576,3 +576,12 @@ func TestContainerCenterFetchAllAndOne(t *testing.T) {
 		assert.Equal(t, true, container.deleteFlag)
 	}
 }
+
+func TestInitClientMutiTime(t *testing.T) {
+	containerCenterInstance = &ContainerCenter{}
+	for i := 0; i < 100; i++ {
+		err := containerCenterInstance.initClient()
+		assert.NotNil(t, err)
+		assert.Nil(t, containerCenterInstance.client)
+	}
+}