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