Fix NPE in ee9/ee8 nested.BufferedResponesHandler

[joakime]

Jetty version(s)
12.1.0

Jetty Environment
ee8 and ee9

HTTP version
Any

Java version/vendor (use: java -version)
Any

OS type/version
Any

Description
If using the nested.BufferedResponseHandler in ee8/ee9 it is possible to trigger a NPE if the following sequence of events occur.

Note: this is not an issue present in the Jetty Core org.eclipse.jetty.server.handler.BufferedResponseHandler (a different class)

new HttpServlet()
{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException
    {
        resp.setStatus(400);
        resp.flushBuffer();
        resp.getOutputStream().close();
    }
}

1. Don't write response content.
2. Set the status code.
3. Flush the response buffers.
4. Manually close the response outputstream.

This messes up the BufferedResponseHandler internal state tracking if an aggregate buffer has been used or not.