Fix: you could join an AI company in multiplayer via the GUI (#9369)

src/network/network_gui.cpp

@@ -1894,7 +1894,7 @@ private:
 
 		if (_network_server) this->buttons[line_count].emplace_back(new CompanyButton(SPR_ADMIN, STR_NETWORK_CLIENT_LIST_ADMIN_COMPANY_TOOLTIP, COLOUR_RED, company_id, &NetworkClientListWindow::OnClickCompanyAdmin, company_id == COMPANY_SPECTATOR));
 		this->buttons[line_count].emplace_back(chat_button);
-		if (own_ci->client_playas != company_id) this->buttons[line_count].emplace_back(new CompanyButton(SPR_JOIN, STR_NETWORK_CLIENT_LIST_JOIN_TOOLTIP, COLOUR_ORANGE, company_id, &NetworkClientListWindow::OnClickCompanyJoin));
+		if (own_ci->client_playas != company_id) this->buttons[line_count].emplace_back(new CompanyButton(SPR_JOIN, STR_NETWORK_CLIENT_LIST_JOIN_TOOLTIP, COLOUR_ORANGE, company_id, &NetworkClientListWindow::OnClickCompanyJoin, company_id != COMPANY_SPECTATOR && Company::Get(company_id)->is_ai));
 
 		this->line_count += 1;