add return statements

these two should return from the current call.

https://github.com/TryStatsN/StatsN/blob/master/src/StatsN/Udp.cs#L46
https://github.com/TryStatsN/StatsN/blob/master/src/StatsN/Udp.cs#L50

This isn't an issue since `GetIpAddressAsync` is wrapped in an try catch, but we shouldn't even bother.