xen/pciback: Do not dereference psdev during printk when it is NULL.

.. instead use BUG_ON() as all the callers of the kill_domain_by_device
check for psdev.
Suggested-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -512,12 +512,7 @@ static void kill_domain_by_device(struct pcistub_device *psdev)
 	int err;
 	char nodename[PCI_NODENAME_MAX];
 
-	if (!psdev) {
-		dev_err(&psdev->dev->dev,
-			"device is NULL when do AER recovery/kill_domain\n");
-		return;
-	}
-
+	BUG_ON(!psdev);
 	snprintf(nodename, PCI_NODENAME_MAX, "/local/domain/0/backend/pci/%d/0",
 		psdev->pdev->xdev->otherend_id);