Customize: WP_Customize_Manager::save_changeset_post() can fail if get_changeset_post_data method returns WP_Error

[westonruter]

If a changeset post is corrupted, then \WP_Customize_Manager::get_changeset_post_data() will return a WP_Error. This method is used by \WP_Customize_Manager::save_changeset_post() to obtain an existing changeset's post data to merge on top of. However, it is not doing the necessary check to see if it is returning a WP_Error, and thus results in a fatal error:

PHP Fatal error: Cannot use object of type WP_Error as array in wp-includes/class-wp-customize-manager.php on line 2334

Will amend [38810] for #30937.

[westonruter]

​https://github.com/xwp/wordpress-develop/pull/238

[westonruter]

In 41012:

Customize: Prevent edge case fatal error when attempting to save changes to a changeset that had previously been corrupted.

Check return value of WP_Customize_Manager::get_changeset_post_data() and return if error instead of assuming it is an array.

Amends [38810].
See #30937.
Fixes #41252.