deregister_globals() does not work correctly when $_COOKIE['GLOBALS'] is specified

When e.g. $_COOKIE['GLOBALS']=1 is sent, the deregister_globals() function calls unset() on $GLOBALS['GLOBALS'] destroying the $GLOBALS array.

This renders the board unusable when register_globals (which was removed in PHP 5.4.0) is set to On.

This was previously reported in https://tracker.phpbb.com/browse/SECURITY-172 but since phpBB does not rely on deregister_globals() but always defines variables on its own, this is not a security issue.