[!44] [!42] [vertex] Initialize vertex_ member. Fixes compiler warning.

Merge branch 'cherry-pick-44a0b570' into 'releases/2.6'

Merge branch 'fix/initialize-vertex-member' into 'master'

See merge request [[!42]][1]

(cherry picked from commit 44a0b570)

dc455b4a \[vertex\] Initialize vertex_ member. Fixes compiler warning.

See merge request [!44]

  [!42]: gitlab.dune-project.org/extensions/dune-foamgrid/merge_requests/42
  [1]: Noneextensions/dune-foamgrid/merge_requests/42
  [!44]: gitlab.dune-project.org/extensions/dune-foamgrid/merge_requests/44

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