Fix compiler warning
Fix compiler warning

signed unsigned compiler warning

keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK