vtkCompositePolyDataMapper2: avoid a NULL dereference
vtkCompositePolyDataMapper2: avoid a NULL dereference

Change-Id: I3c349342efd0be2c9b57f2754aab491324edbbbd

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