Make sure remove_node fully detaches the node

Right now remove_node is only used in contexts where parent is reset after
removing but this might be important in the future.

src/pugixml.cpp

@@ -711,6 +711,7 @@ PUGI__NS_BEGIN
 		else
 			parent->first_child = node->next_sibling;
 
+		node->parent = 0;
 		node->prev_sibling_c = 0;
 		node->next_sibling = 0;
 	}