Network Admin: Properly check if the theme is active on the main site to avoid deleting it via bulk actions.

props ocean90.
fixes #29861.
Built from https://develop.svn.wordpress.org/trunk@29839


git-svn-id: http://core.svn.wordpress.org/trunk@29603 1a063a9b-81f0-0310-95a4-ce76da25c4cd