ipa-kdb: make sure we don't produce MS-PAC in case of authdata flag cleared by admin
When admin clears authdata flag for the service principal, KDC will pass
NULL client pointer (service proxy) to the DAL driver.
Make sure we bail out correctly.
Reviewed-By: Tomáš Babej <tbabej@redhat.com>
Reviewed-By: Simo Sorce <ssorce@redhat.com>