Zero initialize values in vtkGradientFilter
Zero initialize values in vtkGradientFilter

This fixes a "Conditional jump or move depends on uninitialised
value(s)" error from valgrind by zero initializing the values at
the top of ComputeGradientsSG().

Change-Id: Idfc249ed797c798a9bd79ece2883ca48d9735afb

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