merge r15585 from branch 2.3 to trunk

bug 2648 fixed: pwg.categories.getList, avoid to throw an error when
representative_picture_id is missing for an album, which makes parsing
the JSON output impossible.



git-svn-id: http://piwigo.org/svn/trunk@15586 68402e56-0260-453c-a942-63ccdbb3a9ee

include/ws_functions.inc.php

@@ -845,7 +845,7 @@ SELECT id, path, representative_ext
   {
     foreach ($categories as $category)
     {
-      if ($category['id'] == $cat['id'])
+      if ($category['id'] == $cat['id'] and isset($category['representative_picture_id']))
       {
         $cat['tn_url'] = $thumbnail_src_of[$category['representative_picture_id']];
       }