BUG: Fix compile error introduced when backporting fix for MGH origin issue

The compile issue hasn't been detected earlier because only a visual check
between the original patch (Slicer/itkMGHImageIO@13f5e11ef) and
the back-ported one has been done (r22485).

This commit fixes the build issue and also confirm that the associated
Slicer tests now pass:

// --------------
Start 545: py_nomainwindow_test_slicer_mgh
[...]
test_saveShortCompressedNode (test_slicer_mgh.SlicerUtilLoadSaveMGHTests) ... ok
test_saveUnCompressedNode (test_slicer_mgh.SlicerUtilLoadSaveMGHTests) ... ok
// --------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22486 3bd1e089-480b-0410-8dfb-8563597acbee