PCI: OF: Don't crash when bridge parent is NULL.

In pcibios_get_phb_of_node(), we will crash while booting if
bus->bridge->parent is NULL.

Check for this case and avoid dereferencing the NULL pointer.
Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/of.c

@@ -55,7 +55,7 @@ struct device_node * __weak pcibios_get_phb_of_node(struct pci_bus *bus)
 	 */
 	if (bus->bridge->of_node)
 		return of_node_get(bus->bridge->of_node);
-	if (bus->bridge->parent->of_node)
+	if (bus->bridge->parent && bus->bridge->parent->of_node)
 		return of_node_get(bus->bridge->parent->of_node);
 	return NULL;
 }