Adds a missing string and some additional validation of paths in the upgrader class. Follow up to [56992]. Props costdev, jipmoors, karlijnbok, swissspidy, afragen, mukesh27. Fixes #59712. Built from https://develop.svn.wordpress.org/trunk@58022 git-svn-id: http://core.svn.wordpress.org/trunk@57488 1a063a9b-81f0-0310-95a4-ce76da25c4cd