Fix PHP Notice on post save when any of the supported sidebars have been unregistered

If a theme or plugin has unregistered one of the supported sidebars, e.g. `unregister_sidebar( 'sidebar-alt' );` (as many custom themes do), there's a PHP Notice on every post save:

```PHP Notice:  Undefined index: _ss_sidebar_alt in /Sites/www/wp-content/plugins/genesis-simple-sidebars/includes/inpost.php on line 84```

This checks if the $_POST values are set first before assigning them to the array and fixes the notice.