Pass $post_id arg directly to get_post_thumbnail_id() from...

Pass $post_id arg directly to get_post_thumbnail_id() from has_post_thumbnail(). The null/get_the_ID() check is already performed at that level of the stack. see #19012.

git-svn-id: https://core.svn.wordpress.org/trunk@19029 1a063a9b-81f0-0310-95a4-ce76da25c4cd