e_shell_quit() called twice at the end of application

This could exhibit as a double-prompt whether wants to quit with
pending messages in Outbox, where the second prompt, the one with
the EShellWindow being destroyed, didn't make much sense.

shell/e-shell.c

@@ -84,6 +84,7 @@ struct _EShellPrivate {
 	guint network_available_locked : 1;
 	guint online : 1;
 	guint quit_cancelled : 1;
+	guint ready_to_quit : 1;
 	guint safe_mode : 1;
 };
 
@@ -151,7 +152,8 @@ shell_window_removed_cb (EShell *shell)
 {
 	g_return_if_fail (E_IS_SHELL (shell));
 
-	if (!gtk_application_get_windows (GTK_APPLICATION (shell)))
+	if (!gtk_application_get_windows (GTK_APPLICATION (shell)) &&
+	    !shell->priv->ready_to_quit)
 		e_shell_quit (shell, E_SHELL_QUIT_LAST_WINDOW);
 }
 
@@ -410,9 +412,13 @@ shell_ready_for_quit (EShell *shell,
 	GtkApplication *application;
 	GList *list;
 
+	g_return_if_fail (E_IS_SHELL (shell));
+
 	if (!is_last_ref)
 		return;
 
+	shell->priv->ready_to_quit = TRUE;
+
 	application = GTK_APPLICATION (shell);
 
 	/* Increment the reference count so we can safely emit