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>

arch/s390/kvm/interrupt.c

@@ -329,9 +329,13 @@ static inline unsigned long pending_irqs_no_gisa(struct kvm_vcpu *vcpu)
 
 static inline unsigned long pending_irqs(struct kvm_vcpu *vcpu)
 {
-	return pending_irqs_no_gisa(vcpu) |
-		gisa_get_ipm(vcpu->kvm->arch.gisa_int.origin) <<
-			IRQ_PEND_IO_ISC_7;
+	struct kvm_s390_gisa_interrupt *gi = &vcpu->kvm->arch.gisa_int;
+	unsigned long pending_mask;
+
+	pending_mask = pending_irqs_no_gisa(vcpu);
+	if (gi->origin)
+		pending_mask |= gisa_get_ipm(gi->origin) << IRQ_PEND_IO_ISC_7;
+	return pending_mask;
 }
 
 static inline int isc_to_irq_type(unsigned long isc)