storage: prevent ::next from spinning forever

TODO

@@ -32,7 +32,6 @@ TODO:
 * range sparse_set::push should return a reverse iterator, it's kinda wrong now
 * after non-continuous generation for entity storage:
   - get/reset placeholder to position after saving/loading (avoid long lookup)
-  - basic_storage<...>::next never stops in release when it runs out of entities (well, not an issue probably but still)
   - documentation for reserved entities
 * no-version entities always return true when compared to tombstones (fixed but test needed)
 * storage entity: no emplace/insert, rename and add a fast range-push from above

src/entt/entity/storage.hpp

@@ -957,7 +957,7 @@ class basic_storage<Entity, Entity, Allocator>
         do {
             ENTT_ASSERT(placeholder < underlying_type::traits_type::to_entity(null), "Invalid element");
             entt = underlying_type::traits_type::combine(static_cast<typename underlying_type::traits_type::entity_type>(placeholder++), {});
-        } while(base_type::current(entt) != underlying_type::traits_type::to_version(tombstone));
+        } while(base_type::current(entt) != underlying_type::traits_type::to_version(tombstone) && entt != null);
 
         return entt;
     }