ACPICA: Namespace: add check to avoid null pointer dereference

mainline inclusion
from mainline-5.1-rc4
commit df9271d6
category: bugfix
bugzilla: 14713
CVE: NA

-------------------------------------------------

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>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Yao Hongbo <yaohongbo@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>