Wrong usage of `struct evhttp_request` in trace log

[zieckey]

We cannot use struct evhttp_request* req any more after we called evhttp_send_reply.

struct Response {
    Response(struct evhttp_request* r, const std::string& m)
        : req(r), buffer(nullptr) {
        if (m.size() > 0) {
            buffer = evbuffer_new();
            evbuffer_add(buffer, m.c_str(), m.size());
        }
    }

    ~Response() {
        if (buffer) {
            evbuffer_free(buffer);
            buffer = nullptr;
        }

        // At this time, req is freed by evhttp framework probably.
        // So don't use req any more.
        // LOG_TRACE << "free request " << req->uri;
    }

    struct evhttp_request*  req;
    struct evbuffer* buffer;
};

[zieckey]

Fixed