Check for a non-empty cell before using its points
Check for a non-empty cell before using its points

This fixes a segfault in the vtkQuadricClustering::AddEdges()
method that occurs when an empty cell in encountered.

Change-Id: Ic77236e462262164d21cd94283deec3325b156a2

spthaolt/VTK,candy7393/VTK,cjh1/VTK,sankhesh/VTK,sumedhasingla/VTK,ashray/VTK-EVM,aashish24/VTK-old,hendradarwin/VTK,sumedhasingla/VTK,SimVascular/VTK,sankhesh/VTK,hendradarwin/VTK,msmolens/VTK,keithroe/vtkoptix,demarle/VTK,johnkit/vtk-dev,jmerkow/VTK,msmolens/VTK,aashish24/VTK-old,spthaolt/VTK,hendradarwin/VTK,keithroe/vtkoptix,candy7393/VTK,jmerkow/VTK,candy7393/VTK,mspark93/VTK,keithroe/vtkoptix,gram526/VTK,sumedhasingla/VTK,biddisco/VTK,msmolens/VTK,ashray/VTK-EVM,spthaolt/VTK,demarle/VTK,SimVascular/VTK,biddisco/VTK,hendradarwin/VTK,johnkit/vtk-dev,mspark93/VTK,mspark93/VTK,ashray/VTK-EVM,collects/VTK,berendkleinhaneveld/VTK,candy7393/VTK,demarle/VTK,keithroe/vtkoptix,sankhesh/VTK,demarle/VTK,cjh1/VTK,SimVascular/VTK,jmerkow/VTK,berendkleinhaneveld/VTK,biddisco/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,johnkit/vtk-dev,collects/VTK,aashish24/VTK-old,demarle/VTK,aashish24/VTK-old,berendkleinhaneveld/VTK,hendradarwin/VTK,msmolens/VTK,keithroe/vtkoptix,candy7393/VTK,gram526/VTK,cjh1/VTK,jmerkow/VTK,collects/VTK,ashray/VTK-EVM,jmerkow/VTK,demarle/VTK,aashish24/VTK-old,gram526/VTK,mspark93/VTK,sankhesh/VTK,jmerkow/VTK,SimVascular/VTK,ashray/VTK-EVM,aashish24/VTK-old,SimVascular/VTK,biddisco/VTK,sankhesh/VTK,hendradarwin/VTK,ashray/VTK-EVM,keithroe/vtkoptix,johnkit/vtk-dev,gram526/VTK,johnkit/vtk-dev,spthaolt/VTK,sumedhasingla/VTK,SimVascular/VTK,collects/VTK,mspark93/VTK,ashray/VTK-EVM,keithroe/vtkoptix,cjh1/VTK,candy7393/VTK,johnkit/vtk-dev,sumedhasingla/VTK,msmolens/VTK,cjh1/VTK,SimVascular/VTK,jmerkow/VTK,demarle/VTK,berendkleinhaneveld/VTK,cjh1/VTK,mspark93/VTK,spthaolt/VTK,biddisco/VTK,jmerkow/VTK,candy7393/VTK,gram526/VTK,msmolens/VTK,msmolens/VTK,gram526/VTK,spthaolt/VTK,biddisco/VTK,biddisco/VTK,hendradarwin/VTK,sumedhasingla/VTK,johnkit/vtk-dev,gram526/VTK,sumedhasingla/VTK,demarle/VTK,collects/VTK,spthaolt/VTK,candy7393/VTK,berendkleinhaneveld/VTK,sankhesh/VTK,sumedhasingla/VTK,mspark93/VTK,msmolens/VTK,mspark93/VTK,sankhesh/VTK,collects/VTK,berendkleinhaneveld/VTK,sankhesh/VTK,SimVascular/VTK,keithroe/vtkoptix,gram526/VTK