Should not mark red (i.e. ROM allocated) objects.

src/gc.c

@@ -772,6 +772,7 @@ mrb_gc_mark(mrb_state *mrb, struct RBasic *obj)
 {
   if (obj == 0) return;
   if (!is_white(obj)) return;
+  if (is_red(obj)) return;
   mrb_assert((obj)->tt != MRB_TT_FREE);
   add_gray_list(mrb, &mrb->gc, obj);
 }