safety belt : scan_directory() can return without setting arr to a valid value

mongoose.c

@@ -3639,12 +3639,14 @@ static void send_directory_listing(struct connection *conn, const char *dir) {
               sort_direction, sort_direction, sort_direction);
 
   num_entries = scan_directory(conn, dir, &arr);
+  if (arr) {
       qsort(arr, num_entries, sizeof(arr[0]), compare_dir_entries);
       for (i = 0; i < num_entries; i++) {
         print_dir_entry(&arr[i]);
         NS_FREE(arr[i].file_name);
       }
       NS_FREE(arr);
+  }
 
   write_terminating_chunk(conn);
   close_local_endpoint(conn);