[gtk] MenuShell: Typecheck the instance @ select_first()

[Daniel Boles <dboles src gnome org>]

commit f6ba206d07d82503e8401e7a68783508777348e8
Author: Daniel Boles <dboles src gmail com>
Date:   Tue Apr 17 19:49:24 2018 +0100

    MenuShell: Typecheck the instance @ select_first()
    
    since this is public API and therefore might get junk passed to it.

 gtk/gtkmenushell.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkmenushell.c b/gtk/gtkmenushell.c
index 1126d94620..a69bc0ada8 100644
--- a/gtk/gtkmenushell.c
+++ b/gtk/gtkmenushell.c
@@ -1231,9 +1231,13 @@ void
 gtk_menu_shell_select_first (GtkMenuShell *menu_shell,
                              gboolean      search_sensitive)
 {
-  GtkMenuShellPrivate *priv = menu_shell->priv;
+  GtkMenuShellPrivate *priv;
   GList *tmp_list;
 
+  g_return_if_fail (GTK_IS_MENU_SHELL (menu_shell));
+
+  priv = menu_shell->priv;
+
   tmp_list = priv->children;
   while (tmp_list)
     {