[EMailBrowser] Free MessageList on close

There was left a MessageList instance from within the EMailBroser
whenever it was closed, due to it adding a reference to the message
list but not removing it properly.

mail/e-mail-browser.c

@@ -587,7 +587,7 @@ mail_browser_dispose (GObject *object)
 	if (priv->message_list != NULL) {
 		/* This will cancel a regen operation. */
 		gtk_widget_destroy (priv->message_list);
-		priv->message_list = NULL;
+		g_clear_object (&priv->message_list);
 	}
 
 	/* Chain up to parent's dispose() method. */