[gtk+] scale: Fix a crash

[Matthias Clasen <matthiasc src gnome org>]

commit 0024358c91e59b9d6af69e4be7f914e6721129eb
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Nov 3 23:35:32 2015 -0500

    scale: Fix a crash
    
    We can only free the marks_node if there is one.

 gtk/gtkscale.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkscale.c b/gtk/gtkscale.c
index 97f96a6..9cb0d36 100644
--- a/gtk/gtkscale.c
+++ b/gtk/gtkscale.c
@@ -1542,8 +1542,11 @@ gtk_scale_clear_marks (GtkScale *scale)
   gtk_style_context_remove_class (context, GTK_STYLE_CLASS_SCALE_HAS_MARKS_BELOW);
   gtk_style_context_remove_class (context, GTK_STYLE_CLASS_SCALE_HAS_MARKS_ABOVE);
 
-  gtk_css_node_set_parent (priv->marks_node, NULL);
-  priv->marks_node = NULL;
+  if (priv->marks_node)
+    {
+      gtk_css_node_set_parent (priv->marks_node, NULL);
+      priv->marks_node = NULL;
+    }
 
   _gtk_range_set_stop_values (GTK_RANGE (scale), NULL, 0);