Fix second callback when filtering caption text for shortcode, props SergeyBiryukov, fixes #20369

git-svn-id: http://svn.automattic.com/wordpress/trunk@20376 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/media.php

@@ -179,7 +179,7 @@ function _cleanup_image_add_caption($str) {
 
 // Private, preg_replace callback used in image_add_caption()
 function _cleanup_image_add_caption2($str) {
-	return str_replace(	"'", ''', $str );
+	return ( isset( $str[0] ) ) ? str_replace( "'", ''', $str[0] ) : '';
 }
 
 /**