race / crash in CheckAlive

```
goroutine 508 [running]:
github.com/pufferffish/wireproxy.VirtualTun.pingIPs.func1()
        github.com/pufferffish/wireproxy/routine.go:478 +0x5c4
created by github.com/pufferffish/wireproxy.VirtualTun.pingIPs in goroutine 54
        github.com/pufferffish/wireproxy/routine.go:438 +0x71c
```

just crashed for me. 
i know how to fix, will push a PR by the end of the day