Posts, Post Types: Avoid a PHP warning when `get_the_content()` is called outside of the loop.

This ensures that `$pages` and other globals are only used after they have been set up in `setup_postdata()`.

Follow-up to [44941].

Props tessawatkinsllc, dontdream, spacedmonkey, squarecandy, davidbaumwald, SergeyBiryukov.
Fixes #47824. See #42814.
Built from https://develop.svn.wordpress.org/trunk@48114


git-svn-id: https://core.svn.wordpress.org/trunk@47883 1a063a9b-81f0-0310-95a4-ce76da25c4cd