Don't segfault on unset LDAP parameters.
Don't segfault on unset LDAP parameters.

When ldapserver / ldap_uri was specified, but not for example
user_attr, authorize_user_token_ldap() used to cause a segmentation
fault.

madrat-/yubico-pam,madrat-/yubico-pam,Yubico/yubico-pam,Yubico/yubico-pam,madrat-/yubico-pam,eworm-de/yubico-pam,eworm-de/yubico-pam,Yubico/yubico-pam,eworm-de/yubico-pam