goroutine leak

[artemovechkin]

Hi!
When using the package, I encountered the problem of goroutines leakage, when closing the tunnel in hard-to-repeat cases, it apparently happens that the /knx/tunnel.go:358 pushInbound function writes to nil chan

Which in turn, instead of panic and completing the goroutine, there is a lock on nil chan

I would like to correct this point, can I open a pull request with the addition of closing the channel conn.inbound when closing the tunnel /knx/tunnel.go:623

thank you for your attention