GPO: error out instead of leaving array element uninitialized
In general every object created by the AD provider should have a SID
attribute. Since SIDs and GPOs are used for access control a missing SID
should be treated as error for now until it is known if there is a valid
reason why the SID is missing.
Resolves https://fedorahosted.org/sssd/ticket/2608
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit 4cfab2330323834574c179f774a0c6b1fff4936e)