Fix for "Undefined property: WP_Query::$post" in register_globals

[chrisjean]

The register_globals function (wp-includes/classes.php) has an unchecked property reference. This causes the following warning to be generated on post type listings when the post type has no entries:

Notice: Undefined property: WP_Query::$post in ABSPATH/wp-includes/classes.php on line 443

The attached patch fixes removes the warning with an isset check that assigns a value of null if $wp_query->post isn't set. This is the same result as assigning the undefined value without the warning.

[westi]

(In [15750]) Avoid Notice if the query doesn't have a post set e.g. on a 404 . Fixes #15059 props chrisbliss18.