Fix warning in pwg.categories.php

"Undefined variable: total_images" message when request category does not exist

include/ws_functions/pwg.categories.php

@@ -22,6 +22,7 @@ function ws_categories_getImages($params, &$service)
 
   $images = array();
   $image_ids = array();
+  $total_images = 0;
 
   //------------------------------------------------- get the related categories
   $where_clauses = array();