[POWERPC] Add null pointer check to of_find_property

Update function of_find_property() to return NULL if the device_node
passed to it is also NULL.  Otherwise, passing NULL will cause a null
pointer dereference.

Without this, the legacy_serial driver will crash if there's no
'chosen' node in the device tree.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

drivers/of/base.c

@@ -65,6 +65,9 @@ struct property *of_find_property(const struct device_node *np,
 {
 	struct property *pp;
 
+	if (!np)
+		return NULL;
+
 	read_lock(&devtree_lock);
 	for (pp = np->properties; pp != 0; pp = pp->next) {
 		if (of_prop_cmp(pp->name, name) == 0) {