lib-auth: Fix memory leak in auth_client_request_abort()

This caused memory leaks when authentication was aborted. For example
with IMAP:

a AUTHENTICATE PLAIN
*

Broken by 9137c55

src/lib-auth/auth-client-request.c

@@ -180,6 +180,7 @@ void auth_client_request_abort(struct auth_client_request **_request)
 
 	auth_client_send_cancel(request->conn->client, request->id);
 	call_callback(request, AUTH_REQUEST_STATUS_ABORT, NULL, NULL);
+	pool_unref(&request->pool);
 }
 
 unsigned int auth_client_request_get_id(struct auth_client_request *request)