ProtocolServer: Don't crash on failed request

The CNetworkJob::on_finish hook will be invoked both for success and
failure, but there will only be a m_job->response() in the success case
so we have to null-check it before using it.

This should have been obvious from the "->"

Servers/ProtocolServer/HttpDownload.cpp

@@ -7,7 +7,8 @@ HttpDownload::HttpDownload(PSClientConnection& client, NonnullRefPtr<CHttpJob>&&
  , m_job(job)
 {
  m_job->on_finish = [this](bool success) {
- set_payload(m_job->response()->payload());
+ if (m_job->response())
+ set_payload(m_job->response()->payload());
  did_finish(success);
  };
 }