Fix GridMap memory leak

(cherry picked from commit 5d4a141c97)

modules/gridmap/grid_map.cpp

@@ -791,6 +791,7 @@ void GridMap::_update_octants_callback() {
 	}
 
 	while (to_delete.front()) {
+		memdelete(octant_map[to_delete.front()->get()]);
 		octant_map.erase(to_delete.front()->get());
 		to_delete.pop_front();
 	}