WP_Query::have_posts should work when "fields" is set

[wonderboymusic]

If you set fields to ids or id=>parent in a WP_Query instance, WP_Query::post_count does not get set, so WP_Query::have_posts() returns false. It shouldn't.

To test:

$q = new WP_Query( array( 'posts_per_page' => -1 ));
var_dump( $q->have_posts() );

$q = new WP_Query( array( 'posts_per_page' => -1, 'fields' => 'ids' ) );
var_dump( $q->have_posts() );
exit();

You get this for free just by setting WP_Query::post_count - patch attached

dupe of #21147