Fix kernel NULL pointer dereference in xen-blkfront

When booting Xen Dom0 on a pre-release 3.2.1 hypervisor the system Oopses on a
"Unable to handle kernel NULL pointer dereference" in xenwatch.

From the backtrace it looks like backend_changed is calling bdget_disk
with a NULL pointer.  Checking for NULL and returning ENODEV instead
allows the kernel to boot.

drivers/block/xen-blkfront.c

@@ -977,6 +977,8 @@ static void backend_changed(struct xenbus_device *dev,
 		break;
 
 	case XenbusStateClosing:
+		if (info->gd == NULL)
+			xenbus_dev_fatal(dev, -ENODEV, "gd is NULL");
 		bd = bdget_disk(info->gd, 0);
 		if (bd == NULL)
 			xenbus_dev_fatal(dev, -ENODEV, "bdget failed");