LZ4's block streams inherit Closeable.  Closeable's contract specifies:

    /**
     * Closes this stream and releases any system resources associated
     * with it. If the stream is already closed then invoking this
     * method has no effect.
     *
     * @throws IOException if an I/O error occurs
     */
    public void close() throws IOException;

Show that double-close is broken in a way that violates the contract.
Ensure that double-closing BlockOutputStream is not harmful.
Merge pull request #16 from stevenschlansker/double-close

Double close causes exception, which violates Closeable contract.