Fix access of non existing variable backup.inc.php

in file server/lib/classes/backup.inc.php
The restoreBackupWebFiles method gets passed a $filename
In one branche it is getting concatenated into $full_filename
This fixes the variable name from $full_filename to $filename on line 451
Line 451 is outside the branch described above

Author: cztk

server/lib/classes/backup.inc.php

@@ -448,7 +448,7 @@ public static function restoreBackupWebFiles($backup_format, $password, $backup_
                 }
             }
         } else {
-            $app->log('Failed to restore web backup ' . $full_filename . ', backup mode "' . $backup_mode . '" not recognized.', LOGLEVEL_DEBUG);
+            $app->log('Failed to restore web backup ' . $filename . ', backup mode "' . $backup_mode . '" not recognized.', LOGLEVEL_DEBUG);
         }
         $app->system->web_folder_protection($web_root, true);
         return $result;