BUG: Fix uninitialized value in vtkSlicerVolumesLogic.

This commit fix the following error reported by valgrind memcheck tool:

==1290==
==1290== Conditional jump or move depends on uninitialised value(s)
==1290== at 0x3E5C64FA: vtkSlicerVolumesLogic::SetCompareVolumeGeometryEpsilon(double) (vtkSlicerVolumesLogic.cxx:1042)
==1290== by 0x3E5C35BE: vtkSlicerVolumesLogic::vtkSlicerVolumesLogic() (vtkSlicerVolumesLogic.cxx:472)
==1290== by 0x3E5C22B4: vtkSlicerVolumesLogic::New() (vtkSlicerVolumesLogic.cxx:279)
==1290== by 0x436D557F: qSlicerVolumesModule::createLogic() (qSlicerVolumesModule.cxx:167)
==1290== by 0x5D60DC1: qSlicerAbstractCoreModule::logic() (qSlicerAbstractCoreModule.cxx:280)
==1290== by 0x5D603AD: qSlicerAbstractCoreModule::initialize(vtkSlicerApplicationLogic*) (qSlicerAbstractCoreModule.cxx:99)
==1290== by 0x5D928A7: qSlicerModuleFactoryManager::loadModule(QString const&, QString const&) (qSlicerModuleFactoryManager.cxx:170)
==1290== by 0x5D92759: qSlicerModuleFactoryManager::loadModule(QString const&, QString const&) (qSlicerModuleFactoryManager.cxx:157)
==1290== by 0x5D92414: qSlicerModuleFactoryManager::loadModule(QString const&) (qSlicerModuleFactoryManager.cxx:110)
==1290== by 0x406B81: (anonymous namespace)::SlicerAppMain(int, char**) (Main.cxx:231)
==1290== by 0x407213: main (Main.cxx:292)
==1290==

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24433 3bd1e089-480b-0410-8dfb-8563597acbee