Avoid PHP notices in `get_post_type_archive_template()`.

The function should fail more gracefully when called in the context
where `get_query_var( 'post_type' )` doesn't represent an actual post
type.

Props technopolitica, dlh.
Fixes #38374.
Built from https://develop.svn.wordpress.org/trunk@40031


git-svn-id: http://core.svn.wordpress.org/trunk@39968 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/template.php

@@ -135,8 +135,9 @@ function get_post_type_archive_template() {
 		$post_type = reset( $post_type );
 
 	$obj = get_post_type_object( $post_type );
-	if ( ! $obj->has_archive )
+	if ( ! ( $obj instanceof WP_Post_Type ) || ! $obj->has_archive ) {
 		return '';
+	}
 
 	return get_archive_template();
 }

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-40030';
+$wp_version = '4.8-alpha-40031';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.