ACPICA: Namespace: add check to avoid null pointer dereference

ACPICA commit 7586a625f9c34c3169efd88470192bf63119e31a

Some ACPICA userspace tools call acpi_ut_subsystem_shutdown() during
cleanup and dereference a null pointer when cleaning up the
namespace.

Link: https://github.com/acpica/acpica/commit/7586a625


Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>