Fix: Set proper leafChild on construction.

The leafChild was not set if the given element was not a leaf
element but its first descendant (in the sense of hbegin) was
a leaf element.
Presumably, this was introduced in 3c65f9ab

dune/fufem/utilities/hierarchicleafiterator.hh

@@ -56,6 +56,8 @@ public:
 
             if (!hIt_->isLeaf())
                 increment();
+            else
+              leafChild_ = *hIt_;
         }
     }