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>