fix possible null pointer reference in acpi_pci_irq_enable

diff -puN drivers/acpi/pci_irq.c~fix_acpi_pci_irq_enable drivers/acpi/pci_irq.c
@@ -395,6 +395,11 @@ acpi_pci_irq_enable (
 	if (!dev)
 		return_VALUE(-EINVAL);
 	
+	if (!dev->bus) {
+		ACPI_DEBUG_PRINT((ACPI_DB_ERROR, "Invalid (NULL) 'bus' field\n"));
+		return_VALUE(-ENODEV);
+	}
+
 	pci_read_config_byte(dev, PCI_INTERRUPT_PIN, &pin);
 	if (!pin) {
 		ACPI_DEBUG_PRINT((ACPI_DB_INFO, "No interrupt pin configured for device %s\n", pci_name(dev)));

@@ -402,11 +407,6 @@ acpi_pci_irq_enable (
 	}
 	pin--;
 
-	if (!dev->bus) {
-		ACPI_DEBUG_PRINT((ACPI_DB_ERROR, "Invalid (NULL) 'bus' field\n"));
-		return_VALUE(-ENODEV);
-	}
-
 	/* 
 	 * First we check the PCI IRQ routing table (PRT) for an IRQ.  PRT
 	 * values override any BIOS-assigned IRQs set during boot.