Compiler warning from velocityoutput.hh

dumux/dumux/porousmediumflow/velocityoutput.hh:309:109: warning: left operand of comma operator has no effect [-Wunused-value]
                         if (Dune::FloatCmp::eq<NumEqVector>, Dune::FloatCmp::CmpStyle::absolute>(neumannFlux, NumEqVector(0.0), 1e-30))
                                                                                                  ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
dumux/dumux/porousmediumflow/velocityoutput.hh:309:60: warning: left operand of comma operator is a reference, not call, to function ‘Dune::FloatCmp::eq<Dune::FieldVector<double, 3> >’ [-Waddress]
                         if (Dune::FloatCmp::eq<NumEqVector>, Dune::FloatCmp::CmpStyle::absolute>(neumannFlux, NumEqVector(0.0), 1e-30))

g++ (SUSE Linux) 7.3.1 20180817 [gcc-7-branch revision 263612]