$pages global not updated with multiple WP_Query objects/post loops

[misterbisson]

setup_postdata() in wp-includes/query.php globalizes the $pages var, then assigns it to either $pages = explode(...) or $pages[0] = $post->post_content depending on whether the $post->post_content is paginated.

However, the var is not properly reset when displaying posts in additional post loops on a given page that is paginated. As coded, only $pages[0] is reset, leaving other elements of the array unchanged. The attached patch corrects the problem.

(In [14750]) Fix pages setup in setup_postdata(). Props misterbisson. fixes #12651