Prevent ArrayObject recursion in 5.6

In PHP 5.6(next), calling `isset()` on an instance of `ArrayObject` will invoke both `offsetExists()` and `offsetGet()`. Adding an `isset()` inside `offsetGet()` will therefore result in an infinite recursion; this is related to [php-src/614](https://github.com/php/php-src/pull/614).

It causes `tests/ZendTest/StdLib/ParametersTest.php` to fail.

This patch addresses the above issue in a backward compatible manner.