Multithreaded BagOfVisualWords: "memory corrupt" problem

```C#
var Bow = BagOfVisualWords.Create(500);
Bow.ParallelOptions.MaxDegreeOfParallelism = 8;
Bow.Learn(TrainImages.ToArray());

```
`TrainImages` read from file: 
```C#
Bitmap image = ((Bitmap)Accord.Imaging.Image.FromFile(imageFileName));
```
Error: attempted read or write to protected memory

If I set `MaxDegreeOfParallelism=1`, it's all OK.
