Fix crash of vtkPointSetCellIterator
When vtkPointSetCellIterator gets a vtkPointSet with a NULL vtkPoints member it would crash because it would try to set the vtkPointSet datatype. Now this is avoided if the vtkPointSet has a NULL vtkPoints.
When vtkPointSetCellIterator gets a vtkPointSet with a NULL vtkPoints member it would crash because it would try to set the vtkPointSet datatype. Now this is avoided if the vtkPointSet has a NULL vtkPoints.