In set_theme_mod() account for when there is no old value.

fixes #14721. see [27393].

Built from https://develop.svn.wordpress.org/trunk@27402


git-svn-id: http://core.svn.wordpress.org/trunk@27249 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -913,7 +913,7 @@ function get_theme_mod( $name, $default = false ) {
  */
 function set_theme_mod( $name, $value ) {
 	$mods = get_theme_mods();
-	$old_value = $mods[ $name ];
+	$old_value = isset( $mods[ $name ] ) ? $mods[ $name ] : false;
 
 	/**
 	 * Filter the theme mod value on save.