only log protocol name if it is present (6a92621 and 402d3f3 from 0.9)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.8@1943 ef36b2f9-881f-0410-afb5-c4e39611909c