of: Make of_fwnode_handle() safer

On the expense of a little bit more complexity in the of_fwnode_handle()
macro, make the macro result in NULL in case its argument is NULL while
still referencing it only once.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Rob Herring <robh@kernel.org>

include/linux/of.h

@@ -159,7 +159,13 @@ static inline struct device_node *to_of_node(struct fwnode_handle *fwnode)
 		container_of(fwnode, struct device_node, fwnode) : NULL;
 }
 
-#define of_fwnode_handle(node) (&(node)->fwnode)
+#define of_fwnode_handle(node)						\
+	({								\
+		typeof(node) __of_fwnode_handle_node = (node);		\
+									\
+		__of_fwnode_handle_node ?				\
+			&__of_fwnode_handle_node->fwnode : NULL;	\
+	})
 
 static inline bool of_have_populated_dt(void)
 {