Use math lib for underfloweding

[alexisgayte]

https://github.com/brianmcmichael/dss-vest/blob/f808e8a942305dcc3409f00cf320d06283a761d8/src/DssVest.sol#L77

Or block.timestamp need to be casted as well

[brianmcmichael]

None of these values should be able to underflow. See the init which ensures fin is always gt bgn and also no more than 5 years out.

[alexisgayte]

Wrote on the other issue,
the issue is more that the code is not correct whatever if time is 5 years or not. if you cast block.timestamp it becomes correct as you will roll over with the top expression too.

(block.timestamp - _award.bgn) * WAD / (_award.fin - _award.bgn)

(_award.fin - _award.bgn) can go negative while (block.timestamp - _award.bgn) will stay positive.
you get a negative result.

Ok it is not underflow you got the point but the issue is still here.