Submitted by hansfriese, also found by Trust, unforgiven, and cccz
If finalLongPayout is changed twice by admin fault, the market would be insolvent as it should pay more collateral than it has.
If finalLongPayout is less than MAX_PAYOUT, it means the market is ended and longToken Price = finalLongPayout, shortToken Price = MAX_PAYOUT - finalLongPayout.
So when users redeem their long/short tokens, the total amount of collateral tokens will be the same as the amount that users transferred during mint().
Btw in setFinalLongPayout(), there is no validation that this function cant be called twice and the below scenario would be possible.
When there are several users in the market, some users cant redeem their long/short tokens as the market doesnt have enough collaterals.
We should modify setFinalLongPayout() like below so it cant be finalized twice.
ramenforbreakfast (prePO) confirmed 
