[gtk] set: Fix gtk_set_get_max

[Matthias Clasen <matthiasc src gnome org>]

commit 17ea03eee2194d609f68619faf8d7f8ad81feb59
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jun 4 21:35:53 2020 -0400

    set: Fix gtk_set_get_max
    
    This function had an off-by-one error. Thankfully,
    our selection tests are thorough enough to catch it.

 gtk/gtkset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkset.c b/gtk/gtkset.c
index b4e0ae7627..18fab23c56 100644
--- a/gtk/gtkset.c
+++ b/gtk/gtkset.c
@@ -364,7 +364,7 @@ gtk_set_get_max (GtkSet *set)
 
   r = &g_array_index (set->ranges, Range, set->ranges->len - 1);
 
-  return r->first + r->n_items;
+  return r->first + r->n_items - 1;
 }
 
 #if 0