Fix Undefined array key warning

E_WARNING: Undefined array key 3 in ./inc/media.php(1700)
E_WARNING: Trying to access array offset on value of type null in ./inc/media.php(1700)

This happens when the media manager is called with ns parameter set to
a value higher than the last namespace defined in the wiki.

Fixes dokuwiki#4276

Author: dregad

inc/media.php

@@ -1697,7 +1697,7 @@ function media_nstree($ns)
         else $tmp_ns = $part;
 
         // find the namespace parts or insert them
-        while ($data[$pos]['id'] != $tmp_ns) {
+        while (array_key_exists($pos, $data) && $data[$pos]['id'] != $tmp_ns) {
             if (
                 $pos >= count($data) ||
                 ($data[$pos]['level'] <= $level + 1 && Sort::strcmp($data[$pos]['id'], $tmp_ns) > 0)