Read one byte of /favicon.ico before using it

That's probably not enough, but at least fixes sites that simply return
an empty response.

go/favicon/favicon.go

	if resp.StatusCode >= 400 || resp.Header.Get("Content-Length") == "0" {
		return "", errors.New("no /favicon.ico")
	}
	buf := make([]byte, 0, 1)
	n, err := resp.Body.Read(buf)
	if err != nil || n == 0 {
		return "", errors.New("can't read /favicon.ico")
	}
	fmt.Println("favicon.ico", resp.Request.URL.String(), faviconUrl)
	return resp.Request.URL.String(), nil
}