[gtk/a11y-crash: 1/2] scrolledwindow: Fix a11y support code

[Matthias Clasen <matthiasc src gnome org>]

commit 7398323b820ea7d1c432e3a0642d9c32531b6be4
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Aug 4 11:00:32 2020 -0400

    scrolledwindow: Fix a11y support code
    
    This error was introduced by me in 971ee2d28a65b816050c
    and causes a crash in the scrolling benchmark in gtk-demo.
    
    Fixes: #3014

 gtk/gtkscrolledwindow.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index a2f8fcaf4e..7af5ce0563 100644
--- a/gtk/gtkscrolledwindow.c
+++ b/gtk/gtkscrolledwindow.c
@@ -4195,7 +4195,10 @@ gtk_scrolled_window_set_child (GtkScrolledWindow *scrolled_window,
                     NULL);
     }
 
-  list = g_list_append (NULL, priv->child);
+  if (priv->child)
+    list = g_list_append (NULL, priv->child);
+  else
+    list = NULL;
   gtk_accessible_update_relation (GTK_ACCESSIBLE (priv->hscrollbar),
                                   GTK_ACCESSIBLE_RELATION_CONTROLS, list,
                                   -1);