MDL-48929 repository_filesystem: Valide relative path against realpath

On Windows systems, there could have been a mix of back and forward
slashes, causing the validation of the relative path to fail. Now
we will always get the realpath before comparing.

repository/filesystem/lib.php

@@ -575,7 +575,7 @@ class repository_filesystem extends repository {
             $fullrelativefilepath = realpath($this->get_rootpath().$basepath.$relativepath);
 
             // Sanity check to make sure this path is inside this repository and the file exists.
-            if (strpos($fullrelativefilepath, $this->get_rootpath()) === 0 && file_exists($fullrelativefilepath)) {
+            if (strpos($fullrelativefilepath, realpath($this->get_rootpath())) === 0 && file_exists($fullrelativefilepath)) {
                 send_file($fullrelativefilepath, basename($relativepath), null, 0);
             }
         }
@@ -665,4 +665,4 @@ function repository_filesystem_cron() {
             $instances[$itemid]->remove_obsolete_thumbnails($files);
         }
     }
-}
+}