KVM: s390: fix possible null pointer dereference in pending_irqs()

Assure a GISA is in use before accessing the IPM to avoid a
null pointer dereference issue.

Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
Reported-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190131085247.13826-16-mimu@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>