[security] Fixed a issue where hundreds of Warning "first argument of wpdb::prepare should have a placeholder" are displayed while define('WP_DEBUG', true); 

Hi, I was recently using your plugin whilst debugging a multi-site configuration and noticed several small errors and possible vulnerabilities in your plugin. Please see my change log for more information, fix is simple and only 3 lines.

I was doing a little debugging with WordPress 3.9 using your plugin and notice that when your plugin is enabled and wp_debug is set to true. Certain pages on the admin side can generate hundreds possibly thousands of warning for "first argument of wpdb::prepare should have a placeholder".

Summary of the change:
- Orignial: $wpdb->get_col($wpdb->prepare("SELECT domain FROM " . $wpdb->blogs, NULL));
- New:      $wpdb->get_col($wpdb->prepare("SELECT domain FROM %d", $wpdb->blogs));

For information see the ticket that introduced the notice in WP3.9 https://core.trac.wordpress.org/ticket/25604



My configuration:
- WP3.9.1
- Multisite enabled
- WordPress-HTTPS was networked enabled 

Anyway I hope that helps!

Thanks and kind regards,

Steven