[Modify] Lock it

websocket-sharp/Net/HttpListener.cs

@@ -772,6 +772,12 @@ namespace WebSocketSharp.Net
     {
       CheckDisposed ();
 
+      if (_listening)
+        return;
+
+      lock (_contextRegistrySync) {
+        CheckDisposed ();
+
         if (_listening)
           return;
 
@@ -779,6 +785,7 @@ namespace WebSocketSharp.Net
 
         _listening = true;
       }
+    }
 
     /// <summary>
     /// Stops receiving incoming requests.