COMP: Fix sign-compare warning in vtkSlicerVolumesLogic

This commit fixes are a warning introduced by r23934 (BUG: avoid false
positives on geometry check)

// -----------
/path/to/Slicer/Modules/Loadable/Volumes/Logic/vtkSlicerVolumesLogic.cxx:96:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (int i = 0; i < fractional.length(); ++i)
^
/path/to/Slicer/Modules/Loadable/Volumes/Logic/vtkSlicerVolumesLogic.cxx:114:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (i == fractional.length() - 1)
// ----------- ^

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23947 3bd1e089-480b-0410-8dfb-8563597acbee