Themes: Ensure the theme slug is set before checking if the theme is installed or active.

This fixes an issue with every installed theme being marked as active on Themes screen.

The slug is set on Add Themes screen, but not on Themes.

Follow-up to [47924].

Props BackuPs, mukesh27, SergeyBiryukov.
Fixes #50381. See #50334.
Built from https://develop.svn.wordpress.org/trunk@48037


git-svn-id: https://core.svn.wordpress.org/trunk@47804 1a063a9b-81f0-0310-95a4-ce76da25c4cd