Close crashes if server is not running

[briansorahan]

Found this due to stupid behavior on my part, but thought it might be good to be more defensive in the OscServer.Close method. Here is the code that reproduces:

package main

import (
    "github.com/hypebeast/go-osc/osc"
)

func main() {
    server := osc.NewOscServer("127.0.0.1", 23898)
    server.Close()
}

[hypebeast]

This issue is fixed with #5