ACPI, APEI, EINJ, cleanup 0 vs NULL confusion

This function is returning pointers.  Sparse complains here:
drivers/acpi/apei/einj.c:262:32: warning:
	Using plain integer as NULL pointer

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>