[!56] [!55] Properly initialize members of LeafIndexSet

Merge branch 'cherry-pick-26dd3047' into 'releases/2.6'

ref:extensions/dune-foamgrid Merge branch 'fix/leafindexset-intitializer-list'
into 'master'

See merge request [[extensions/dune-foamgrid!55]][1]

(cherry picked from commit 26dd3047)

26c6ee2c Properly initialize members of LeafIndexSet

See merge request [extensions/dune-foamgrid!56]

  [extensions/dune-foamgrid!55]: gitlab.dune-project.org/extensions/dune-foamgrid/merge_requests/55
  [1]: Noneextensions/dune-foamgrid/merge_requests/55
  [extensions/dune-foamgrid!56]: gitlab.dune-project.org/extensions/dune-foamgrid/merge_requests/56

dune/foamgrid/foamgrid/foamgridindexsets.hh

@@ -207,7 +207,8 @@ class FoamGridLeafIndexSet :
 public:
 
     /** \brief Copy constructor */
-    FoamGridLeafIndexSet(const GridImp& g): grid_(g)
+    FoamGridLeafIndexSet(const GridImp& g)
+    : grid_(g), numQuads_(0), numTriangles_(0), numEdges_(0), numVertices_(0)
     {}
 
         //! get index of an entity