fixes #291, add function categories_integrity

to check and repair integrity on categories. For example,
image_category.category_id should always be linked to a categorie.id,
if not we delete the orphan line.

admin/maintenance.php

@@ -69,6 +69,7 @@ switch ($action)
   case 'categories' :
   {
     images_integrity();
+    categories_integrity();
     update_uppercats();
     update_category('all');
     update_global_rank();