s390/vmur: fix irq pointer dereference in int handler

"irq" in vmur's int handler can be an error pointer. Don't dereference
this pointer in that case.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/vmur.c

@@ -306,10 +306,11 @@ static void ur_int_handler(struct ccw_device *cdev, unsigned long intparm,
 {
 	struct urdev *urd;
 
-	TRACE("ur_int_handler: intparm=0x%lx cstat=%02x dstat=%02x res=%u\n",
-	      intparm, irb->scsw.cmd.cstat, irb->scsw.cmd.dstat,
-	      irb->scsw.cmd.count);
-
+	if (!IS_ERR(irb)) {
+		TRACE("ur_int_handler: intparm=0x%lx cstat=%02x dstat=%02x res=%u\n",
+		      intparm, irb->scsw.cmd.cstat, irb->scsw.cmd.dstat,
+		      irb->scsw.cmd.count);
+	}
 	if (!intparm) {
 		TRACE("ur_int_handler: unsolicited interrupt\n");
 		return;