ACPICA: Validate start object for acpi_walk_namespace.

Perform a sanity check on the start object to prevent problems
later. ACPICA BZ 1025.
This patch only adds additional input parameter validation, no actual
kernel suffering has been discovered.

Buglink: http://bugs.acpica.org/show_bug.cgi?id=1025
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>