[PATCH] tun: darwin: fix panic: close of closed channel

[刘海潮]

I used the previous version, so this bug was triggered. But why should we close tun.events in two places.

> 在 2019年8月25日，23:22，Jason A. Donenfeld <Jason at zx2c4.com> 写道：
> 
> I was under the impression this was fixed with
> https://git.zx2c4.com/wireguard-go/commit/?id=6d78f895579b114c93bd493ad8d61316fc5dd15a
> 
> What precisely is the bug you're seeing?