openldap: Fix plugin shutdown with connection failed

openldap: Fix plugin shutdown with connection failed

When plugin failes to connect to LDAP server, the `ldap_unbind_ext_s(st->ld)` is called.
According to `man 3 ldap_unbind_ext_s`, that call is used to free the resources contained in the ld structure.
When plugin shutdown, the `ldap_unbind_ext_s` called again, which causes coredump.

The plugin code was changed to set st->ld to NULL after `ldap_unbind_ext_s()` call.