Bugfix: wrote out of bounds. It took gcc 4.8 and -O2 -g to trigger a segfault...

Bugfix: wrote out of bounds. It took gcc 4.8 and -O2 -g to trigger a segfault that could be tracked down by valgrind. Approved by Thomas.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10359 2fb0f335-1f38-0410-981e-8018bf24f1b0