BUG: Fix crash by ensuring private member is initialized.

Thanks to Bill Lorensen, Alex Yarmarkovich and Csaba Pinter for pointing
out the problem.

To provide some background:

While working on the commit r21482 that introduced the problem, the
variable was automatically initialized to 0 on my workstattion and I didn't
face the issue.

Having valgrind reporting uninitialized variable on the dashboard would
also be helpful.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21491 3bd1e089-480b-0410-8dfb-8563597acbee