unix: req queue must be empty when destroying loop

Assert that the request queue is empty when destroying the event loop.

Should catch errors where people call uv_loop_delete() when there are
still in-progress work requests (for example.)

src/unix/loop.c

@@ -101,6 +101,7 @@ void uv__loop_delete(uv_loop_t* loop) {
 
   uv_mutex_lock(&loop->wq_mutex);
   assert(QUEUE_EMPTY(&loop->wq) && "thread pool work queue not empty!");
+  assert(!uv__has_active_reqs(loop));
   uv_mutex_unlock(&loop->wq_mutex);
   uv_mutex_destroy(&loop->wq_mutex);