ACPICA: Fix possible dereference of null pointer

Fix dereference of possibly null pointer "Predefined" in the case
where the method is not one of the predefined methods.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>