Don't call WP_Query::get_posts() directly

[Chouby]

WP_Query::get_posts() should not be called directly because the WP_Query constructor calls it indirectly through its call to WP_Query:query().

See: ​https://github.com/WordPress/WordPress/blob/5.4.2/wp-includes/class-wp-query.php#L3539-L3543 and ​https://github.com/WordPress/WordPress/blob/5.4.2/wp-includes/class-wp-query.php#L3432

At best calling WP_Query::get_posts() makes a useless second database query. At worst, the results are wrong as the query is not correctly initialized.

[Chouby]

The proposed patch removes the call to WP_Query::get_posts() and replaces it by directly accessing to $query->posts.

[swissspidy]

Good catch!

[swissspidy]

In 48282:

Sitemaps: Do not unnecessarily call WP_Query::get_posts() in posts provider.

The posts have already been fetched at this point, no need to do it again.

Props Chouby.
Fixes #50463.