Fix signed subtraction overflow in CBigNum::setint64().
Fix signed subtraction overflow in CBigNum::setint64().

As noticed by sipa (Pieter Wuille), this can happen when CBigNum::setint64() is
called with an integer value of INT64_MIN (-2^63).

When compiled with -ftrapv, the program would crash. Otherwise, it would
execute an undefined operation (although in practice, usually the correct one).

ligerzero459/paycoin,PaycoinFoundation/paycoin,thommyV91/Suck-a-Coin,PaycoinFoundation/paycoin,LambocoinFoundation/Lambocoin,thommyV91/Suck-a-Coin,LambocoinFoundation/Lambocoin,mitchellcash/paycoin,mitchellcash/paycoin,mitchellcash/paycoin,mitchellcash/paycoin,IngCr3at1on/paycoin,PaycoinFoundation/paycoin,LambocoinFoundation/Lambocoin,LambocoinFoundation/Lambocoin,ligerzero459/paycoin,thommyV91/Suck-a-Coin,thommyV91/Suck-a-Coin,PaycoinFoundation/paycoin,ligerzero459/paycoin,IngCr3at1on/paycoin,ligerzero459/paycoin,IngCr3at1on/paycoin,IngCr3at1on/paycoin