Update auth.php

In Novell eDir the group search returns strings, not arrays. Added if-statement which determines if the result is an array or an string.
Update auth.php

As suggested by @selfthinker
Merge branch 'master' of git://github.com/SteScho/dokuwiki into pull-request-524

* 'master' of git://github.com/SteScho/dokuwiki:
  Update auth.php
  Update auth.php
authldap: handle bad groupkey gracefully
Merge branch 'pull-request-524'

* pull-request-524:
  authldap: handle bad groupkey gracefully
  Update auth.php
  Update auth.php