Fix the failing `Tests_Link::test_wp_get_shortlink()` assertion:

* `wp_get_shortlink()` was firing a notice when reading `$post->ID` while `$post` was null in some cases
* Before the assertions that assume `$GLOBALS['post']` is not set, call `unset( $GLOBALS['post'] );` - there was global spillage from other tests

See #25282.



Built from https://develop.svn.wordpress.org/trunk@25404


git-svn-id: http://core.svn.wordpress.org/trunk@25334 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/link-template.php

@@ -2417,6 +2417,7 @@ function wp_get_shortlink($id = 0, $context = 'post', $allow_slugs = true) {
 		$post = get_post( $post_id );
 	} elseif ( 'post' == $context ) {
 		$post = get_post( $id );
+		if ( ! empty( $post->ID ) )
 			$post_id = $post->ID;
 	}