If the `get_the_excerpt()` is called out of bounds of a global `$post`, return ''.

Fixes #22413.

Built from https://develop.svn.wordpress.org/trunk@28880


git-svn-id: http://core.svn.wordpress.org/trunk@28679 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post-template.php

@@ -341,6 +341,9 @@ function get_the_excerpt( $deprecated = '' ) {
 		_deprecated_argument( __FUNCTION__, '2.3' );
 
 	$post = get_post();
+	if ( empty( $post ) ) {
+		return '';
+	}
 
 	if ( post_password_required() ) {
 		return __( 'There is no excerpt because this is a protected post.' );