[kernel] rt_object_delete函数优化重置object->type

object->type = 0;
改为
object->type = RT_Object_Class_Null;
一个东西，但是提高源码可读性

src/object.c

@@ -485,7 +485,7 @@ void rt_object_delete(rt_object_t object)
     RT_OBJECT_HOOK_CALL(rt_object_detach_hook, (object));
 
     /* reset object type */
-    object->type = 0;
+    object->type = RT_Object_Class_Null;
 
     /* lock interrupt */
     temp = rt_hw_interrupt_disable();