Withdraw more than expected causes underflow about yearn-vaults

[Macarse]

yHegic strategies are also doing that same fix:
https://github.com/Macarse/yhegic/blob/master/contracts/strategies/StrategyHegic.sol#L102-L103

[banteg]

A better fix suggested by Sam
banteg/strategy-uni-lp-pickle@08f849a

[fubuloubu]

It is intended that the implementer be aware of the amount freed, and that it should exceed neither the amount needed nor the amount the strategy currently has available to it locally. It is only intended to use this function in withdraw but we can discuss further of how it could be used in other functions to reduce duplicated code in strategies.

https://github.com/iearn-finance/yearn-vaults/blob/cff924f1894cca1820a588b14d341c4fa4f384c0/contracts/BaseStrategy.sol#L373-L379