No need to check for 0 address on setDesignatedToken

[auryn-macmillan]

https://github.com/gnosis/SafeExit/blob/69e8e5bc7c21b4f027f65f9a3efe4d2a85d7a701/contracts/SafeExit.sol#L160
I don't think there is any need for this check.
Sure, the contract won't work if it's set to 0x0, but that's the same for the vast majority of addresses.
The important thing is that if it is accidentally set to 0x0, or any other invalid address, is that it won't brick the contract.