Networks and Sites: Check the result of get_site() in wp_insert_site().

Props davidbinda, vanyukov, lukecarbis.
Merges [44799] to the 5.1 branch.
Fixes #46300.

git-svn-id: https://develop.svn.wordpress.org/branches/5.1@44800 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-site.php

@@ -69,6 +69,10 @@ function wp_insert_site( array $data ) {
 
 	$new_site = get_site( $wpdb->insert_id );
 
+	if ( ! $new_site ) {
+		return new WP_Error( 'get_site_error', __( 'Could not retrieve site data.' ) );
+	}
+
 	clean_blog_cache( $new_site );
 
 	/**