ACPICA: Avoid NULL pointer arithmetic

We should not assume NULL is defined as "(void *)0" because NULL is
an implementation-defined macro.  Especially, Clang 6 complains about
it, i.e., "arithmetic on a null pointer treated as a cast from integer
to pointer is a GNU extension".

Signed-off-by: Jung-uk Kim <jkim@free_BSD.org>
Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>