[Fix] Lock it early

websocket-sharp/Net/HttpListener.cs

@@ -560,7 +560,11 @@ namespace WebSocketSharp.Net
       if (!_listening)
         throw new InvalidOperationException ("The listener hasn't been started.");
 
-      // Lock _waitQueue early to avoid race conditions.
+      // Lock _ctxRegistrySync early to avoid race conditions.
+      lock (_ctxRegistrySync) {
+        if (!_listening)
+          throw new InvalidOperationException ("The listener is stopped/closed.");
+
         lock (_waitQueueSync) {
           var ctx = getContextFromQueue ();
           if (ctx != null) {
@@ -570,6 +574,7 @@ namespace WebSocketSharp.Net
 
           _waitQueue.Add (asyncResult);
         }
+      }
 
       return asyncResult;
     }