ACPICA: Remove extraneous check for null walk_state

ACPICA commit f9fd6e8bad0f16ce2b436c5cda36ced0c2d85302

Reported by Markus Elfring.

Link: https://github.com/acpica/acpica/commit/f9fd6e8b


Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>