Return to step 1 on custom-header.php if not POST data is sent. Good for if...

Return to step 1 on custom-header.php if not POST data is sent. Good for if step=2 or 3 is in the URL and the page is refreshed (otherwise a nonce check fails). see #20249.

git-svn-id: https://core.svn.wordpress.org/trunk@20211 1a063a9b-81f0-0310-95a4-ce76da25c4cd