ACPICA: Fix possible negative array index in acpi_ut_validate_exception

Added NULL fields to the exception string arrays to eliminate
the -1 subtraction on the SubStatus field.

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>
Signed-off-by: Andi Kleen <ak@linux.intel.com>