[gtk+] a11y: Handle a treeview with no columns

[Benjamin Otte <otte src gnome org>]

commit e0d360233141e4d4fb22d5d332e5372560daff20
Author: Benjamin Otte <otte redhat com>
Date:   Mon Nov 13 03:51:53 2017 +0100

    a11y: Handle a treeview with no columns
    
    Code was spewing criticals to stderr because of nonexisting columns. So
    check that there's actually an existing column first.

 gtk/a11y/gtktreeviewaccessible.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gtk/a11y/gtktreeviewaccessible.c b/gtk/a11y/gtktreeviewaccessible.c
index 44d68fe..3a184e7 100644
--- a/gtk/a11y/gtktreeviewaccessible.c
+++ b/gtk/a11y/gtktreeviewaccessible.c
@@ -1890,7 +1890,7 @@ static GtkTreeViewColumn *
 get_effective_focus_column (GtkTreeView       *treeview,
                             GtkTreeViewColumn *column)
 {
-  if (column == NULL)
+  if (column == NULL && get_n_columns (treeview) > 0)
     column = get_visible_column (treeview, 0);
 
   return column;