Don't allow text injection in our 404 page

We got a report on HackerOne by sumitthehacker:

i want to report a text injection and a misconfiguration of the 404 page

the bug exists at :

https://www.torproject.org/test/%2f../It%20has%20been%20changed%20by%20a%20new%20one%20https://www.Attacker.com%20so%20go%20to%20the%20new%20one%20since%20this%20one

as you can see attacker text is included
"It has been changed by a new one https://www.attacker.com so go to the new one since this one was not found on this server."