Networks and Sites: check if the BLOGUPLOADDIR constant is defined before using it.

This change prevents a fatal PHP error from occurring when `ms-files.php` is accessed without legacy multisite file-handling support being configured.

Instead of the fatal error, these same requests will new return a header status `500` along with a `die()` message "500 - Directory not configured." This differs from the existing `404` statuses to help indicate to site owners this fallback is not working as intended.

Props danielpw, dd32, jakeparis, johnjamesjacoby, spacedmonkey, saurabh.dhariwal, sukhendu2002, realloc.

Fixes #63104.

git-svn-id: https://develop.svn.wordpress.org/trunk@60956 602fd350-edb4-49c9-b593-d223f7449a82

Author: JJJ

src/wp-includes/ms-files.php

@@ -25,6 +25,11 @@
 	die( '404 — File not found.' );
 }
 
+if ( ! defined( 'BLOGUPLOADDIR' ) ) {
+	status_header( 500 );
+	die( '500 — Directory not configured.' );
+}
+
 $file = rtrim( BLOGUPLOADDIR, '/' ) . '/' . str_replace( '..', '', $_GET['file'] );
 if ( ! is_file( $file ) ) {
 	status_header( 404 );