[gtk] a11y: return -1 if parent is NULL

[Samuel Thibault <sthibaul src gnome org>]

commit 2fa9f934b670560d1e3d2522cb90b3be3b76790e
Author: Christian Hergert <chergert redhat com>
Date:   Fri Nov 19 11:59:29 2021 -0800

    a11y: return -1 if parent is NULL

 gtk/a11y/gtkatspicontext.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/gtk/a11y/gtkatspicontext.c b/gtk/a11y/gtkatspicontext.c
index b6bd77b694..43d40ee257 100644
--- a/gtk/a11y/gtkatspicontext.c
+++ b/gtk/a11y/gtkatspicontext.c
@@ -334,6 +334,9 @@ get_index_in_parent (GtkWidget *widget)
   GtkWidget *child;
   int idx;
 
+  if (parent == NULL)
+    return -1;
+
   idx = 0;
   for (child = gtk_widget_get_first_child (parent);
        child;