async/server: Fix the the dropping of the server object

Break the accepting-new-connection loop correctly when the server object
was dropped.

Fixes: #284

Signed-off-by: Tim Zhang <tim@hyper.sh>

Author: Tim-Zhang

src/asynchronous/server.rs

@@ -162,8 +162,8 @@ impl Server {
                     fd_tx = stop_listen_rx.recv() => {
                         if let Some(fd_tx) = fd_tx {
                             fd_tx.send(incoming).await.unwrap();
-                            break;
                         }
+                        break;
                     }
                 }
             }