fixed out-of-bounds array access caught by clang ubsan

It complained:
vtkOrderedTriangulator.cxx:1609  runtime error: index 8 out of bounds for type 'TemplateIDType [8]'

Fixed by checking that cellType is VTK_HEXAHEDRON
(Thanks to Will Schroeder!) so that we don't get
into the problematic code path.

Change-Id: Id737f5c317f6fe8cde6800774f1ffe6f5d32aef6
Merge topic 'vtkOrderedTriangulator-out-of-bounds' into master

791a4f21 fixed out-of-bounds array access caught by clang ubsan