ipadb_mspac_get_trusted_domains: NULL ptr deref
    
    Fix potential NULL pointer deref in ipadb_mspac_get_trusted_domains().
    In theory, dn could be empty and rdn NULL. The man page for ldap_str2dn()
    does not guarantee that it returns a non-empty result.
    
    See: https://pagure.io/freeipa/issue/7738
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>