Fixing a notice when using get_the_image in admin

When using get_the_image in the admin, for example in an admin page, the $post object might (probably won't) be an object or even set, so this simple check will ensure it is an object and the notice is not thrown.

Notice: Trying to get property of non-object in /home/ryan/www/nexus/thenexus/wp-includes/post-template.php on line 29