Avoid a PHP notice when using WP_Query::get_queried_object() in pre_get_posts action.

props wpsmith.
fixes #28412.

git-svn-id: https://develop.svn.wordpress.org/trunk@28667 602fd350-edb4-49c9-b593-d223f7449a82
Move is_admin check to a more appropriate place.

see #11330.

git-svn-id: https://develop.svn.wordpress.org/trunk@28668 602fd350-edb4-49c9-b593-d223f7449a82