BUG Fix issue with Session::destroy crashing after session_write_close

This only came up when using the https://github.com/silverstripe-labs/silverstripe-hybridsessions module where Session::destroy was called after a session_write_close in a request filter.

The problem is that session_write_close doesn't unset the session_id, but session_destroy does.

There is php 5.3 compatible code in here that relies on an error suppression operator.