A servo dying like that could happen at full steam, and then NO ordinary failsafe will save you...
A Servo return spring attached directly to the car thottle linkage might help, that is also needed if you go racing, to avoid runaways in case of a broken throttle servo rod.
I had it happen at full speed, and did not have a strong throttle return spring....
Luckily my car flipped over and landed on the roof, before hurting anyone or anything...
Another option would be an Engine Kill failsafe, or a remotely controlled engine kill switch. Those kan be found here:
http://www.tune-fg.de
Sorry, that site is in german only.