Fix: GC must visit the 'iter' field of every BoxedIterWrapper

src/runtime/iterobject.cpp

@@ -63,6 +63,7 @@ static void iterwrapperGCVisit(GCVisitor* v, Box* b) {
     boxGCHandler(v, b);
 
     BoxedIterWrapper* iw = static_cast<BoxedIterWrapper*>(b);
+    v->visit(iw->iter);
     if (iw->next)
         v->visit(iw->next);
 }

src/runtime/iterobject.h

@@ -45,7 +45,7 @@ public:
     Box* iter;
     Box* next;
 
-    BoxedIterWrapper(Box* iter) : iter(iter) {}
+    BoxedIterWrapper(Box* iter) : iter(iter), next(NULL) {}
 
     DEFAULT_CLASS(iterwrapper_cls);
 };