io.Writer implementation needs a nil check

[pradhanbv]

To reproduce bug, just remove one of the echo server run command from example.sh, but keep the port in backend list. tcptee will crash if it is told to write to a port nobody is listening on.

I have fixed by adding nil check, this will handle backends which are not always available.

// Write implements the io.Writer. func (b Backends) Write(p []byte) (n int, err error) { for _, conn := range b { if conn != nil { n, err = conn.Write(p) if err != nil { return n, err } } } return }