Avoid "use of uninitialized member" warning

Initialize grid-impl pointer to nullptr in default
constructor to avoid compiler warning.

dune/grid/uggrid/uggridleafiterator.hh

@@ -68,7 +68,7 @@ namespace Dune {
 
     /** \brief Constructor setting up an 'end' iterator
      */
-    UGGridLeafIterator()
+    UGGridLeafIterator() : grid_(nullptr)
     {
       entity_.impl().setToTarget(nullptr,nullptr);
     }

dune/grid/uggrid/uggridleveliterator.hh

@@ -35,7 +35,7 @@ namespace Dune {
     enum {codimension = codim};
 
     //! Constructor
-    explicit UGGridLevelIterator()
+    explicit UGGridLevelIterator() : gridImp_(nullptr)
     {
       entity_.impl().setToTarget(nullptr,nullptr);
     }