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