fix advection boundary

fluid/fluid_problem.c

@@ -237,7 +237,7 @@ static void f_boundary(WeakBoundary *wbnd, FluidProblem *problem,const double *n
     if (unbnd - unmesh < 0) {
       for (int id = 0; id < D; ++id) {
         f0[U+id] += ((unbnd - unmesh)*(vid<0?0:uext[id])-(unold - unmesh)*u[id])*rho/c;
-        f00[(U+id)*n_fields+U+id] -= unold*rho/c;
+        f00[(U+id)*n_fields+U+id] -= (unold-unmesh)*rho/c;
       }
     }
   }