fluid states: avoid possible division by zero in massFraction()

if the mean molar mass is zero, then the sum of the mass fractions
will be zero anyway, which means that the result should be still correct.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7095 2fb0f335-1f38-0410-981e-8018bf24f1b0