KVM: Fix KVM_SET_SIGNAL_MASK with arg == NULL

When the user passed in a NULL mask pass this on from the ioctl
handler.

Found by gcc 4.6's new warnings.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/kvm_main.c

@@ -1547,7 +1547,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
 				goto out;
 			p = &sigset;
 		}
-		r = kvm_vcpu_ioctl_set_sigmask(vcpu, &sigset);
+		r = kvm_vcpu_ioctl_set_sigmask(vcpu, p);
 		break;
 	}
 	case KVM_GET_FPU: {