BUG: Fixed vtkNRRDReader for large images

Slicer crashed when nrrd image larger than 2GB was attempted to be loaded.
The issue was that at two places 32-bit variables were used for computing total number of bytes in the image.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25668 3bd1e089-480b-0410-8dfb-8563597acbee