Redirects: Prevent redirects if a queried object exists.

After [34659], it became possible to cause an incorrect redirect, by changing the slug of a post, then creating a new post with the old slug. The correct behaviour is to prevent redirecting to the old post.

Props dd32, pento.

Fixes #35031 for trunk.


Built from https://develop.svn.wordpress.org/trunk@36128


git-svn-id: https://core.svn.wordpress.org/trunk@36094 1a063a9b-81f0-0310-95a4-ce76da25c4cd