COMP: avoid undefined symbol in Qt5 with Q_ASSERT

Apparently in Qt4 built in release mode Q_ASSERT is a noop in
release mode so the compiler never sees the variable.
But Qt5 there is a compile error about missing variable 'd'.

From: Steve Pieper <pieper@isomics.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25885 3bd1e089-480b-0410-8dfb-8563597acbee