Fixed #493 (assertion in MacOS build)

Problem was: default-constructed iterators are not the same in clang's STL

src/db/db/dbHierarchyBuilder.cc

@@ -31,6 +31,9 @@ namespace db
 
 static HierarchyBuilderShapeInserter def_inserter;
 
+static HierarchyBuilder::cell_map_type null_map;
+static HierarchyBuilder::cell_map_type::const_iterator null_iterator = null_map.end ();
+
 // -------------------------------------------------------------------------------------------
 
 int
@@ -168,7 +171,7 @@ HierarchyBuilder::reset ()
   m_cell_map.clear ();
   m_cells_seen.clear ();
   m_cell_stack.clear ();
-  m_cm_entry = cell_map_type::const_iterator ();
+  m_cm_entry = null_iterator;
   m_cm_new_entry = false;
 }
 
@@ -263,14 +266,14 @@ HierarchyBuilder::end (const RecursiveShapeIterator *iter)
   m_cells_seen.clear ();
   mp_initial_cell = m_cell_stack.empty () ? 0 : m_cell_stack.front ().second.front ();
   m_cell_stack.clear ();
-  m_cm_entry = cell_map_type::const_iterator ();
+  m_cm_entry = null_iterator;
   m_cm_new_entry = false;
 }
 
 void
 HierarchyBuilder::enter_cell (const RecursiveShapeIterator * /*iter*/, const db::Cell * /*cell*/, const db::Box & /*region*/, const box_tree_type * /*complex_region*/)
 {
-  tl_assert (m_cm_entry != m_cell_map.end () && m_cm_entry != cell_map_type::const_iterator ());
+  tl_assert (m_cm_entry != m_cell_map.end () && m_cm_entry != null_iterator);
 
   m_cells_seen.insert (m_cm_entry->first);