Yeah, I've done that more than once, left the switch on in the 850 & killed a 4s batt. I don't believe there is any concern with lipo fires from leaving a switch on, running the lipo completely dead. The risk is rather trying to recharge them again & everytime after that!
Lipos are pricy but compared to the damage they could cause if something goes wrong with recharging or during use
.... discard it & get a new one.
I use my .22 rifle to blow up my ruined lipos from a distance, then chuck 'em in the burning barrel.
I've had one burst into flames after shooting it, 3 or 4 others didn't do anything.