Potential fix for a crash in OnThreadExecutor (#5840)

src/common/on_thread_executor.h

@@ -22,10 +22,9 @@ public:
 private:
     void worker_thread();
 
-    std::thread _worker_thread;
-
     std::mutex _task_mutex;
     std::condition_variable _task_cv;
     std::atomic_bool _shutdown_request;
     std::queue<std::packaged_task<void()>> _task_queue;
+    std::thread _worker_thread;
 };