Remove NULL check on reference.
Remove NULL check on reference.

A reference to a class member cannot be NULL, and checking if it is
NULL results in a compiler warning in clang 3.3.

Change-Id: I3bcb0d37ab3a202f2bea92cba0238a5fe96aa2b2

sankhesh/VTK,jmerkow/VTK,sankhesh/VTK,sankhesh/VTK,keithroe/vtkoptix,mspark93/VTK,johnkit/vtk-dev,sumedhasingla/VTK,keithroe/vtkoptix,sankhesh/VTK,candy7393/VTK,sankhesh/VTK,johnkit/vtk-dev,msmolens/VTK,demarle/VTK,johnkit/vtk-dev,jmerkow/VTK,keithroe/vtkoptix,candy7393/VTK,msmolens/VTK,ashray/VTK-EVM,candy7393/VTK,demarle/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,hendradarwin/VTK,sumedhasingla/VTK,msmolens/VTK,keithroe/vtkoptix,msmolens/VTK,gram526/VTK,gram526/VTK,candy7393/VTK,hendradarwin/VTK,mspark93/VTK,msmolens/VTK,jmerkow/VTK,SimVascular/VTK,candy7393/VTK,demarle/VTK,ashray/VTK-EVM,hendradarwin/VTK,demarle/VTK,mspark93/VTK,sumedhasingla/VTK,ashray/VTK-EVM,ashray/VTK-EVM,SimVascular/VTK,gram526/VTK,demarle/VTK,gram526/VTK,mspark93/VTK,ashray/VTK-EVM,SimVascular/VTK,gram526/VTK,mspark93/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,hendradarwin/VTK,johnkit/vtk-dev,mspark93/VTK,SimVascular/VTK,johnkit/vtk-dev,candy7393/VTK,sumedhasingla/VTK,sumedhasingla/VTK,demarle/VTK,jmerkow/VTK,gram526/VTK,sankhesh/VTK,johnkit/vtk-dev,jmerkow/VTK,demarle/VTK,hendradarwin/VTK,jmerkow/VTK,candy7393/VTK,sumedhasingla/VTK,ashray/VTK-EVM,jmerkow/VTK,sankhesh/VTK,johnkit/vtk-dev,SimVascular/VTK,mspark93/VTK,gram526/VTK,hendradarwin/VTK,keithroe/vtkoptix,gram526/VTK,keithroe/vtkoptix,keithroe/vtkoptix,mspark93/VTK,demarle/VTK,candy7393/VTK,ashray/VTK-EVM,msmolens/VTK,sankhesh/VTK,hendradarwin/VTK,ashray/VTK-EVM,msmolens/VTK,jmerkow/VTK,msmolens/VTK