Remove use of strnlen.
Remove use of strnlen.

It has issue on certain old MinGW dashboards. Replacing by strlen()
instead, which is safe for our use-case.

keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix