[HUST CSE][vbus] fix the NULL pointer error

components/vbus/vbus_chnx.c

@@ -230,15 +230,16 @@ rt_uint8_t rt_vbus_get_chnnr(rt_device_t dev)
 void rt_vbus_chnx_register_disconn(rt_device_t dev,
                                    rt_vbus_event_listener indi,
                                    void *ctx)
+{
+    if (dev && dev->user_data)
     {
         struct rt_vbus_dev *vdev = dev->user_data;
-
         RT_ASSERT(vdev->chnr != 0);
 
-    if (vdev)
         rt_vbus_register_listener(vdev->chnr, RT_VBUS_EVENT_ID_DISCONN,
                                   indi, ctx);
     }
+}
 
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof(a[0]))