// found.
func findDirectory64End(r io.ReaderAt, directoryEndOffset int64) (int64, error) {
	locOffset := directoryEndOffset - directory64LocLen
	if locOffset < 0 {
		return -1, nil // no need to look for a header outside the file
	}
	buf := make([]byte, directory64LocLen)
	if _, err := r.ReadAt(buf, locOffset); err != nil {
		return -1, err
	}
	b := readBuf(buf)