testentrycompletion: Avoid a compiler cast warning

Instead of getting the model again but then forgetting to cast it to a
ListStore, just use the other variable store where we already did both.

tests/testentrycompletion.c

@@ -233,7 +233,7 @@ animation_timer (GtkEntryCompletion *completion)
 	{
 	  g_print ("removing model!\n");
 
-	  old_store = g_object_ref (gtk_entry_completion_get_model (completion));
+	  old_store = g_object_ref (store);
 	  gtk_entry_completion_set_model (completion, NULL);
 	}
       else