Ensure wp_roles() returns an instance of WP_Roles

[IanDelMar]

The original implementation of wp_roles() only checked whether the global variable $wp_roles was set (isset) before returning it. This could lead to issues if $wp_roles was set to a non-WP_Roles value, resulting in potential errors or unexpected behaviour.

The fix introduces an additional check to ensure that $wp_roles is an instance of WP_Roles. If it is not, a new WP_Roles object is instantiated, ensuring that wp_roles() always returns a valid WP_Roles instance.

Trac ticket: https://core.trac.wordpress.org/ticket/62959

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