Unable to set Custom “Link URL” for Featured Image, and More?

I cleaned up the code u have given and it should work as you intend.

Let me know if your still having problems.

<div id="slider">
<?php
    $c = get_option('wpns_category');
    $n = get_option('wpns_slices');
    $s = new WP_Query( array( 'cat' => $c, 'posts_per_page' => $n ) );
    if( $s->have_posts() ): while( $s->have_posts() ): $s->the_post(); global $post;
?>
        <?php if(has_post_thumbnail()){ ?>
            <a href="https://wordpress.stackexchange.com/questions/33600/<?php echo get_post_meta($post->ID,"slider-url", true); ?>" title="<?php the_title(); ?>">
                <?php the_post_thumbnail(); ?>
            </a>
        <?php } ?>
    <?php endwhile; endif;?>
</div>