gc.c (incremental_marking_phase): clear gcnext for debugging

src/gc.c

@@ -1048,6 +1048,7 @@ incremental_marking_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
   while (gc->gray_list && tried_marks < limit) {
     struct RBasic *obj = gc->gray_list;
     gc->gray_list = obj->gcnext;
+    obj->gcnext = NULL;
     gc_mark_children(mrb, gc, obj);
     tried_marks += gc_gray_counts(mrb, gc, obj);
   }