Check the sidebar class for orphaned sidebars, match sidebars by id while...

Check the sidebar class for orphaned sidebars, match sidebars by id while retrieving orphans and the theme has not changed, props SergeyBiryukov, fixes #19350

git-svn-id: https://core.svn.wordpress.org/trunk@19455 1a063a9b-81f0-0310-95a4-ce76da25c4cd