Fix a memory overrun in vtkImageStencilIterator.
The code called SetSpanState() after incrementing past the end of the stencil, causing a memory read past the end of the allocated memory. This resulted in unpredictable behaviour and random crashes.
The code called SetSpanState() after incrementing past the end of the stencil, causing a memory read past the end of the allocated memory. This resulted in unpredictable behaviour and random crashes.