Mark the current context (not root) in final marking.

src/gc.c

@@ -993,7 +993,7 @@ incremental_marking_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
 static void
 final_marking_phase(mrb_state *mrb, mrb_gc *gc)
 {
-  mark_context(mrb, mrb->root_c);
+  mark_context(mrb, mrb->c);
   gc_mark_gray_list(mrb, gc);
   mrb_assert(gc->gray_list == NULL);
   gc->gray_list = gc->atomic_gray_list;