IOMMU: make domctl handler tolerate NULL domain

Besides the reporter's issue of hitting a NULL deref when !CONFIG_GDBSX,
XEN_DOMCTL_test_assign_device can legitimately end up having NULL passed
here, when the domctl was passed DOMID_INVALID.

Fixes: 71e617a6b8 ("use is_iommu_enabled() where appropriate...")
Reported-by: Cheyenne Wills <cheyenne.wills@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Juergen Gross <jgross@suse.com>

xen/drivers/passthrough/iommu.c

@@ -568,7 +568,7 @@ int iommu_do_domctl(
 {
     int ret = -ENODEV;
 
-    if ( !is_iommu_enabled(d) )
+    if ( !(d ? is_iommu_enabled(d) : iommu_enabled) )
         return -EOPNOTSUPP;
 
 #ifdef CONFIG_HAS_PCI