Fix some issues for chapter-13

[jerryoung]

    /**
     * Returns a slice of this buffer's sub-region. Modifying the content of
     * the returned buffer or this buffer affects each other's content while
     * they maintain separate indexes and marks.
     * This method does not modify {@code readerIndex} or {@code writerIndex} of
     * this buffer.
     * <p>
     * Also be aware that this method will NOT call {@link #retain()} and so the
     * reference count will NOT be increased.
     */
    public abstract ByteBuf slice(int index, int length);

This method does not modify readerIndex or writerIndex of this buffer.
So we should change from ：

        String logMsg = data.slice(idx + 1,
            data.readableBytes()).toString(CharsetUtil.UTF_8);

to:

        String logMsg = data.slice(idx + 1,
            data.readableBytes() - idx - 1).toString(CharsetUtil.UTF_8);

data.readableBytes() - idx - 1 will slice a right length, it will work better.

[good2luck]

nice, solved my problem.