[Modify] Polish it

websocket-sharp/WebSocketFrame.cs

@@ -597,14 +597,15 @@ Extended Payload Length: {7}
       Action<Exception> error
     )
     {
-      var len = frame.IsMasked ? 4 : 0;
+      if (!frame.IsMasked) {
-      if (len == 0) {
         frame._maskingKey = WebSocket.EmptyBytes;
         completed (frame);
 
         return;
       }
 
+      var len = 4;
+
       stream.ReadBytesAsync (
         len,
         bytes => {