When restoring sidebars after switching back to an old theme, make sure the...

When restoring sidebars after switching back to an old theme, make sure the saved data matches the theme's current sidebars, fixes #19092

git-svn-id: https://core.svn.wordpress.org/trunk@19332 1a063a9b-81f0-0310-95a4-ce76da25c4cd