Fix PHP Notice with WP_DEBUG on

Over a year ago I reported this issue - see
https://wordpress.org/support/topic/php-notice-13

It was never fixed, so here is the fix.

Problem:

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