Fix PHP Notice with WP_DEBUG on

PHP Notice: Undefined index: ctslider_slidecaption_1_meta_box_nonce in /var/www/html/wp-content/plugins/captain-slider/includes/admin/meta-box.php on line 143

This is happening on pages without the slider
Merge pull request #16 from krbvroc1/fix_php_errors

Fix PHP Notice with WP_DEBUG on