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
}