Fix hashset iteration bug

src/lib/DataStructures/HashSet.vala

@@ -86,7 +86,7 @@ namespace Invercargill.DataStructures {
         }
 
         public override Tracker<T> get_tracker () {
-            return Iterate.range(0, (int)n_buckets)
+            return Iterate.range(0, buckets.length)
                 .where(i => buckets[i] != null && buckets[i] != tombstone)
                 .select<T>(i => buckets[i]->item)
                 .get_tracker();