of: handle NULL node in next_child iterators

Add an early check for the node argument in __of_get_next_child and
of_get_next_available_child() to avoid dereferencing a NULL node pointer
a few lines after.

CC: Daniel Mack <zonque@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

drivers/of/base.c

@@ -700,6 +700,9 @@ static struct device_node *__of_get_next_child(const struct device_node *node,
 {
 	struct device_node *next;
 
+	if (!node)
+		return NULL;
+
 	next = prev ? prev->sibling : node->child;
 	for (; next; next = next->sibling)
 		if (of_node_get(next))
@@ -746,6 +749,9 @@ struct device_node *of_get_next_available_child(const struct device_node *node,
 	struct device_node *next;
 	unsigned long flags;
 
+	if (!node)
+		return NULL;
+
 	raw_spin_lock_irqsave(&devtree_lock, flags);
 	next = prev ? prev->sibling : node->child;
 	for (; next; next = next->sibling) {