clang ubsan warned running some of the 'EnSightBlow' tests:

IO/EnSight/vtkEnSight6BinaryReader.cxx:2823:15: runtime error: signed integer overflow: -1358823424 * 4 cannot be represented in type 'int'

I just upcast to 64 bit so that the multiplication doesn't overflow.
The tests now pass with the ubsan check.

I have subtly changed the behaviour here, but believe the
intent is preserved.

Change-Id: I0a35789c169204f04d2a683a029e992e9b20ca65