EnSight6BinaryReader: fix overflow byte order setting

The vtkEnSight6BinaryReader uses the number of unstructured points to determine byte ordering the first time they are read into the program. It does this by comparing the value, with both LE ordering and BE ordering, to the file size.

This fix also adds some consistency checks for identifying integer overflow when it occurs.

Backport: release

Backport: paraview/release