Mark the exception object in the final marking phase; ref #3702

src/gc.c

@@ -995,6 +995,7 @@ static void
 final_marking_phase(mrb_state *mrb, mrb_gc *gc)
 {
   mark_context(mrb, mrb->c);
+  mrb_gc_mark(mrb, (struct RBasic*)mrb->exc);
   gc_mark_gray_list(mrb, gc);
   mrb_assert(gc->gray_list == NULL);
   gc->gray_list = gc->atomic_gray_list;