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:
e8c331e963c58b83db24b7d0e39e8c07f687dbc6
"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: James Bottomley <[e-mail]>
Acked-by: Jesse Barnes <[e-mail]>
Signed-off-by: Len Brown <[e-mail]>

drivers/pci/hotplug/acpiphp_glue.c

···
 	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);
 	}