[gnome-shell/wip/fmuellner/return-val-if-fail] st-widget: Add missing g_return_val_if_fail()

[Florian Müllner <fmuellner src gnome org>]

commit 9f81cb862d4e12597aa0f7ff513f496122ee252b
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Mar 5 19:42:01 2019 +0100

    st-widget: Add missing g_return_val_if_fail()
    
    It's a public function, so it's good practice to sanity-check its
    input (unless disabled at compile time of course).
    
    https://gitlab.gnome.org/GNOME/gnome-shell/issues/1018

 src/st/st-widget.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/src/st/st-widget.c b/src/st/st-widget.c
index f5ca8928a..b20148906 100644
--- a/src/st/st-widget.c
+++ b/src/st/st-widget.c
@@ -579,7 +579,11 @@ get_root_theme_node (ClutterStage *stage)
 StThemeNode *
 st_widget_get_theme_node (StWidget *widget)
 {
-  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
+  StWidgetPrivate *priv;
+
+  g_return_val_if_fail (ST_IS_WIDGET (widget), NULL);
+
+  priv = st_widget_get_instance_private (widget);
 
   if (priv->theme_node == NULL)
     {