vtkGeometryRepresentation: Check vtkPVGeometryFilter assumptions

The vtkPointSpriteRepresentation changes GeometryFilter to something
other than vtkPVGeometryFilter causing NULL dereferences here.

Change-Id: I76bce83e7387a1595fdc07be619439947546206a
Merge topic 'point-sprite-plugin-crash' into next

23cdfb17 vtkGeometryRepresentation: Check vtkPVGeometryFilter assumptions