ACPICA: Add a check for a null node during namespace walk.

Mostly for consistency. ACPICA BZ 1042.

Actually, currently no one is experiencing problem without this check
as the obj_handle is guaranteed to be valid.

References: https://bugs.acpica.org/show_bug.cgi?id=1042
Reported-by: Colin Ian King <colin.king@canonical.com>
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>