Deadlock fix

[stenya]

Deadlock observed when starting multiple ping routines. It happens when writting to recv channel (which is full) and no active readers for this channel.

Example to reproduce the issue :

var waiter sync.WaitGroup
pingFunc := func() {
    defer waiter.Done()

    pinger, err := ping.NewPinger("8.8.8.8")
    if err != nil {
        return
    }

    pinger.SetPrivileged(true)
    pinger.Count = 5
    pinger.Interval = time.Second
    pinger.Timeout = time.Second * 4

    pinger.Run()
}

for i := 0; i < 1000; i++ {
    waiter.Add(1)
    go pingFunc()
}

waiter.Wait() // deadlock here! (reproducible almost every time)

[CHTJonas]

Hi @stenya! Would you feel comfortable adding something to the test suite that checks for this deadlock?

[SuperQ]

This is also duplicated by #84.

[SuperQ]

Going to merge this, and see if we can get #84 re-based on top.