ACPI PCI hotplug: harden against panic regression

ACPI hotplug panic with current git head
http://lkml.org/lkml/2009/1/10/136

Rather than reverting the entire commit that causes the crash:
e8c331e9
"PCI hotplug: introduce functions for ACPI slot detection"

simply harden against it while the changes to
the hotplug code on this particularl machine are understood.
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NLen Brown <len.brown@intel.com>

drivers/pci/hotplug/acpiphp_glue.c

@@ -266,6 +266,8 @@ static int detect_ejectable_slots(struct pci_bus *pbus)
 	int found = acpi_pci_detect_ejectable(pbus);
 	if (!found) {
 		acpi_handle bridge_handle = acpi_pci_get_bridge_handle(pbus);
+		if (!bridge_handle)
+			return 0;
 		acpi_walk_namespace(ACPI_TYPE_DEVICE, bridge_handle, (u32)1,
 				    is_pci_dock_device, (void *)&found, NULL);
 	}