[vertex] Initialize vertex_ member. Fixes compiler warning.

dune/foamgrid/foamgrid/foamgridvertex.hh

@@ -123,7 +123,7 @@ namespace Dune {
         std::vector<const FoamGridEntityImp<dimgrid, dimgrid ,dimworld>*> elements_;
 
         //! A vertex array for compatibility reasons with edges. Calling results in undefined behaviour.
-        std::array<const FoamGridEntityImp<0, dimgrid ,dimworld>*, 1> vertex_;
+        std::array<const FoamGridEntityImp<0, dimgrid ,dimworld>*, 1> vertex_ = {{ nullptr }};
 
         //! Boundary index if vertex is on boundary
         //  only used if the vertex is a boundary vertex