[Modify] Polish it

websocket-sharp/Server/WebSocketServer.cs

@@ -558,8 +558,14 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfAvailable (true, false, false);
-        if (msg != null) {
+        string msg;
+        if (!checkIfAvailable (true, false, false, true, out msg)) {
+          _logger.Error (msg);
+          return;
+        }
+
+        lock (_sync) {
+          if (!checkIfAvailable (true, false, false, true, out msg)) {
             _logger.Error (msg);
             return;
           }
@@ -567,6 +573,7 @@ namespace WebSocketSharp.Server
           _userCredFinder = value;
         }
       }
+    }
 
     /// <summary>
     /// Gets or sets the wait time for the response to the WebSocket Ping or Close.