box models: make the valgrind checks more strict

now, every time a quantity is queried from the problem, it is set to
undefined before and afterwards it is checked that it is defined

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6763 2fb0f335-1f38-0410-981e-8018bf24f1b0