avoid one unhandled exception when listener socket quit without inten…

…ded closure

src/SuperSocket.Server/Connection/TcpConnectionListener.cs

@@ -92,7 +92,7 @@ private async Task KeepAccept(Socket listenSocket)
                 }
             }
 
-            _stopTaskCompletionSource.TrySetResult(true);
+            _stopTaskCompletionSource?.TrySetResult(true);
         }
 
         public event NewConnectionAcceptHandler NewConnectionAccept;