use rb_ast_dispose instead of rb_ast_free

`rb_ast_dispose` calls `rb_gc_writebarrier_remember`.  Unless we call
it, the marked objects may not be GC'ed until `rb_ast_t` is GC'ed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60722 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

parse.y

@@ -12109,7 +12109,7 @@ ripper_parse0(VALUE parser_v)
     parser_prepare(parser);
     parser->ast = rb_ast_new();
     ripper_yyparse((void*)parser);
-    rb_ast_free(parser->ast);
+    rb_ast_dispose(parser->ast);
     parser->ast = 0;
     return parser->result;
 }