Fix reverse iterator issue caught by ASAN

This issue could be seen when dragging an arrangement marker beyond the
bottom of the list in the right edge.

IIUC, std::reverse_iterator<Iter>::operator* creates a temporary iterator
which it then dereferences. This is a problem because what we are
derefencing is a Gtk::TreeIter<Gtk::TreeRow>. gtkmm documentation for
Gtk::TreeIter< T >::operator* states, "The returned reference is
implemented by casting from *this, and so the returned reference is only
valid while this iter is." Additionally, cpp documentation for
std::reverse_iterator states, "std::reverse_iterator does not work with
iterators whose dereference returns a reference to a member of *this."

We also are not advancing this iterator at all, so whether it is reverse
or not is irrelevant (we just want the last one). Thus, *prev(rows.end ())
instead of *rows.rbegin ().

gtk2_ardour/editor_sections.cc

@@ -402,7 +402,7 @@ EditorSections::drag_data_received (Glib::RefPtr<Gdk::DragContext> const& contex
 		/* paste at end */
 		TreeModel::Children rows = _model->children ();
 		assert (!rows.empty ());
-		Gtk::TreeModel::Row row = *rows.rbegin ();
+		Gtk::TreeModel::Row row = *prev(rows.end ());
 		to                      = row[_columns.end];
 #ifndef NDEBUG
 		cout << "EditorSections::drag_data_received - paste at end\n";