xen/pciback: Check dev_data before using it

If pcistub_init_device fails, the release function will be called with
dev_data set to NULL.  Check it before using it to avoid a NULL pointer
dereference.
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -106,7 +106,8 @@ static void pcistub_device_release(struct kref *kref)
 	 * is called from "unbind" which takes a device_lock mutex.
 	 */
 	__pci_reset_function_locked(dev);
-	if (pci_load_and_free_saved_state(dev, &dev_data->pci_saved_state))
+	if (dev_data &&
+	    pci_load_and_free_saved_state(dev, &dev_data->pci_saved_state))
 		dev_info(&dev->dev, "Could not reload PCI state\n");
 	else
 		pci_restore_state(dev);