lib-ldap: Use ldap_unbind_ext() instead of ldap_destroy()

This allows it to compile with older LDAP libraries. Their behavior (at least
in OpenLDAP) is identical as long as LDAP_OPT_CLIENT_CONTROLS haven't been
changed, which we haven't.

src/lib-ldap/ldap-connection.c

@@ -572,7 +572,7 @@ void ldap_connection_kill(struct ldap_connection *conn)
 		}
 	}
 	if (conn->conn != NULL) {
-		ldap_destroy(conn->conn);
+		ldap_unbind_ext(conn->conn, NULL, NULL);
 		ldap_memfree(conn->scred);
 	}
 	conn->conn = NULL;