KVM: s390: Check kvm pointer when testing KVM_CAP_S390_HPAGE_1M

KVM allows issuing the KVM_CHECK_EXTENSION ioctl either on the /dev/kvm
fd or the VM fd. In the first case, kvm_vm_ioctl_check_extension() is
called with kvm==NULL. Ensure we don't dereference the pointer in that
case.

Fixes: 40ebdb8e59df ("KVM: s390: Make huge pages unavailable in ucontrol VMs")
Signed-off-by: Jean-Philippe Brucker <[e-mail]>
Reviewed-by: Claudio Imbrenda <[e-mail]>
Reviewed-by: Christian Borntraeger <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Christian Borntraeger <[e-mail]>

arch/s390/kvm/kvm-s390.c

···
 		break;
 	case KVM_CAP_S390_HPAGE_1M:
 		r = 0;
-		if (hpage && !kvm_is_ucontrol(kvm))
+		if (hpage && !(kvm && kvm_is_ucontrol(kvm)))
 			r = 1;
 		break;
 	case KVM_CAP_S390_MEM_OP: