return NSS_STATUS_TRYAGAIN on zero-length (but not-NULL) buffer (thanks Jakub Hrozek) (f21efd6 from 0.9)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.8@1944 ef36b2f9-881f-0410-afb5-c4e39611909c