Sorry for taking a long time to reply. There is a potion shop in the Underground City. I've taken another look the code (it's been a long time since I wrote it!) to make sure that drinking the Potion of Fire is indeed safe:
https://github.com/btco/so12l/blob/master/xl/potion.xl#L132
The logic is: if the character is NOT frozen, they take 30 points of damage; if they are frozen, they defrost and take no damage.