xen/xsm: flask: Prevent NULL deference in flask_assign_{, dt}device()

xen/xsm: flask: Prevent NULL deference in flask_assign_{, dt}device()

flask_assign_{, dt}device() may be used to check whether you can test if
a device is assigned. In this case, the domain will be NULL.

However, flask_iommu_resource_use_perm() will be called and may end up
to deference a NULL pointer. This can be prevented by moving the call
after we check the validity for the domain pointer.

Coverity-ID: 1486741
Fixes: 71e617a6b8 ('use is_iommu_enabled() where appropriate...')
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Paul Durrant <paul@xen.org>
Release-acked-by: Juergen Gross <jgross@suse.com>