Move _show_post_preview() back next to _set_preview(). see #23539.

git-svn-id: http://core.svn.wordpress.org/trunk@23866 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Make sure the autosave post exists before getting its meta. props kovshenin. fixes #23539.

git-svn-id: http://core.svn.wordpress.org/trunk@23867 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix incorrect script handle and a notice. props ocean90. fixes #23896.

git-svn-id: http://core.svn.wordpress.org/trunk@23868 1a063a9b-81f0-0310-95a4-ce76da25c4cd