Fix never succeeding read into empty buffer

The problem was that we set the capacity to one, but the length to zero,
which meant that `Read` could never put any data into the buffer,
returning zero, which meant that favicon thought there was no data.

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)
	buf := make([]byte, 1)
	n, err := resp.Body.Read(buf)
	if err != nil || n == 0 {
		return "", errors.New("can't read /favicon.ico")