[Modify] Polish it

websocket-sharp/WebSocketFrame.cs

@@ -573,13 +573,14 @@ Extended Payload Length: {7}
       Stream stream, WebSocketFrame frame
     )
     {
-      var len = frame.IsMasked ? 4 : 0;
-      if (len == 0) {
+      if (!frame.IsMasked) {
         frame._maskingKey = WebSocket.EmptyBytes;
         return frame;
       }
 
+      var len = 4;
       var bytes = stream.ReadBytes (len);
+
       if (bytes.Length != len) {
         var msg = "The masking key of a frame could not be read.";
         throw new WebSocketException (msg);