acpi: fix integer as NULL pointer warning

drivers/acpi/dispatcher/dsmethod.c:568:50: warning: Using plain integer as NULL pointer
drivers/acpi/executer/exmutex.c:329:30: warning: Using plain integer as NULL pointer
drivers/acpi/executer/exmutex.c:466:31: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>