Set client user_instance under lock to prevent race upon client removal

src/stratifier.c

@@ -2201,6 +2201,7 @@ static user_instance_t *generate_user(ckpool_t *ckp, stratum_instance_t *client,
 		user->id = sdata->user_instance_id++;
 		HASH_ADD_STR(sdata->user_instances, username, user);
 	}
+	client->user_instance = user;
 	DL_FOREACH(user->worker_instances, tmp) {
 		if (!safecmp(workername, tmp->workername)) {
 			client->worker_instance = tmp;
@@ -2411,7 +2412,7 @@ static json_t *parse_authorise(stratum_instance_t *client, const json_t *params_
 		*err_val = json_string("Invalid character in username");
 		goto out;
 	}
-	user = client->user_instance = generate_user(ckp, client, buf);
+	user = generate_user(ckp, client, buf);
 	client->user_id = user->id;
 	ts_realtime(&now);
 	client->start_time = now.tv_sec;