Merge pull request #2364 from take-cheeze/free_atexit_stack

Release `mrb->atexit_stack` on `mrb_close`.

src/state.c

@@ -221,11 +221,13 @@ mrb_free_context(mrb_state *mrb, struct mrb_context *c)
 void
 mrb_close(mrb_state *mrb)
 {
+  if (mrb->atexit_stack_len > 0) {
     mrb_int i;
-
     for (i = mrb->atexit_stack_len; i > 0; --i) {
       mrb->atexit_stack[i - 1](mrb);
     }
+    mrb_free(mrb, mrb->atexit_stack);
+  }
 
   /* free */
   mrb_gc_free_gv(mrb);