You'll buy it dirt cheap to cover your short position and keep the initial capital you got from the short sale. Pretty dangerous though, even if you're right in the long term, short term anything can happen and you'll get liquidated.
Bitcoin could go over a million dollars, or below one dollar, multiple times, in any order. It would be a risky short, even if it did ultimately end up valued at 0.
All fiat currencies go to zero eventually as well. Good luck timing it.