Properly use compare_exchange_strong

In practice, calling `network.scgi.open_port` again doesn't trigger
this function.

src/thread_worker.cc

@@ -67,11 +67,10 @@ ThreadWorker::init_thread() {
 
 bool
 ThreadWorker::set_scgi(rpc::SCgi* scgi) {
-  if (m_scgi != nullptr)
+  rpc::SCgi* expected = nullptr;
+  if (!m_scgi.compare_exchange_strong(expected, scgi))
     return false;
 
-  m_scgi = scgi;
-
   change_xmlrpc_log();
 
   queue_item((thread_base_func)&start_scgi);