use `weak_ptr`'s `lock()` method to allow case of no coarser level

On the coarsest level, `element_->coarser_` is empty and constructing
a `shared_ptr` from a `weak_ptr` fails.  The `lock()` method returns
an empty `shared_ptr` instead.

This fails when iterating over the hierarchy as the end of the
iteration cannot be reached; the problem was previously described by
@simon.praetorius [1].

  [1]: https://lists.dune-project.org/pipermail/dune-devel/2019-March/002441.html

dune/istl/paamg/hierarchy.hh

@@ -170,7 +170,7 @@ namespace Dune
         /** @brief Move to the next coarser level */
         void increment()
         {
-          element_ = std::shared_ptr<Element>(element_->coarser_);
+          element_ = element_->coarser_.lock();
         }
 
         /** @brief Move to the next fine level */