MpNc test problem: start tabulation at 0 pressure

this issue was not triggered because the obstacle problem uses the
H2O-N2 fluid system with the complex fluid relations disabled.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7276 2fb0f335-1f38-0410-981e-8018bf24f1b0