BUG 15704: Fix crash with invalid CGNF file.
BUG 15704: Fix crash with invalid CGNF file.

For bump_hdh5.cgns from http://cgns.sourceforge.net/CGNSFiles.html
CGNS reader cannot read it, ParaView passes it to the MPASReader which
segsfaults.

keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix