Remoce some build warnings

src/list.c

@@ -756,8 +756,7 @@ get_data_as_string (GtkTreeIter *iter)
     }
 
   str->str[str->len-1] = '\0';
-  res = str->str;
-  g_string_free (str, FALSE);
+  res = g_string_free_and_steal (str);
 
   return res;
 }