s390/dasd: check for device error pointer within state change interrupts

Check if the device pointer is valid. Just a sanity check since we already
are in the int handler of the device.
Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd.c

@@ -1712,9 +1712,12 @@ void dasd_int_handler(struct ccw_device *cdev, unsigned long intparm,
 	/* check for for attention message */
 	if (scsw_dstat(&irb->scsw) & DEV_STAT_ATTENTION) {
 		device = dasd_device_from_cdev_locked(cdev);
-		device->discipline->check_attention(device, irb->esw.esw1.lpum);
+		if (!IS_ERR(device)) {
+			device->discipline->check_attention(device,
+							    irb->esw.esw1.lpum);
 			dasd_put_device(device);
 		}
+	}
 
 	if (!cqr)
 		return;