Sssd: sssd: fail-open in ldap ppolicy access check allows continued authorization

A flaw was found in SSSD. When configured with the LDAP access provider and `ldap_access_order` including `ppolicy` or `lockout`, a fail-open condition in the LDAP ppolicy access check can occur if a user lookup returns zero results. This can incorrectly return success and cache an allow decision, permitting continued authorization for a deleted or deprovisioned user. A remote attacker with prior valid account context could exploit this to maintain access to information and potentially make limited modifications to resources that should no longer be available.