Mark the root context in `final_marking_phase`.

Because the root context cannot be marked from fibers.

src/gc.c

@@ -997,6 +997,7 @@ final_marking_phase(mrb_state *mrb, mrb_gc *gc)
 {
   mrb_gc_mark_gv(mrb);
   mark_context(mrb, mrb->c);
+  mark_context(mrb, mrb->root_c);
   mrb_gc_mark(mrb, (struct RBasic*)mrb->exc);
   gc_mark_gray_list(mrb, gc);
   mrb_assert(gc->gray_list == NULL);