Fix logic error
Fix logic error

This could result in a free(NULL) call. This code path can only be
triggered if pam_ldap changes the logged-in username (introduced in
6a74d8d).

Thanks 依云, see
https://github.com/arthurdejong/nss-pam-ldapd/issues/11

arthurdejong/nss-pam-ldapd,arthurdejong/nss-pam-ldapd,arthurdejong/nss-pam-ldapd