gc.c (rb_free_tmp_buffer): stop accessing imemo_alloc as NODE

The fields of imemo_alloc were accessed via RNODE() cast, since the
imemo was NODE_ALLOCA traditionally.  This was refactored at r60239,
so now the fields should be accessed as imemo_alloc.  This prevented
change of NODE structure.

Yuichiro Kaneko pointed out this inconsistency.  Thanks!

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60648 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -8143,10 +8143,10 @@ rb_alloc_tmp_buffer(volatile VALUE *store, long len)
 void
 rb_free_tmp_buffer(volatile VALUE *store)
 {
-    VALUE s = ATOMIC_VALUE_EXCHANGE(*store, 0);
+    rb_imemo_alloc_t *s = (rb_imemo_alloc_t*)ATOMIC_VALUE_EXCHANGE(*store, 0);
     if (s) {
-	void *ptr = ATOMIC_PTR_EXCHANGE(RNODE(s)->u1.node, 0);
-	RNODE(s)->u3.cnt = 0;
+	void *ptr = ATOMIC_PTR_EXCHANGE(s->ptr, 0);
+	s->cnt = 0;
 	ruby_xfree(ptr);
     }
 }