[gtk+] listbox: Fix a crash during keynav

[Matthias Clasen <matthiasc src gnome org>]

commit 4573d2d19b91a1c4152e74b0582d36a8b0f2dc13
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Dec 15 18:46:21 2017 -0500

    listbox: Fix a crash during keynav
    
    The code was asserting something that was not always holding
    true. We can hit row == NULL here on page-up too. Handle that
    case by moving to the first row.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791549

 gtk/gtklistbox.c |   23 +++++++++++++----------
 1 files changed, 13 insertions(+), 10 deletions(-)
---
diff --git a/gtk/gtklistbox.c b/gtk/gtklistbox.c
index a3c638d..6d704c5 100644
--- a/gtk/gtklistbox.c
+++ b/gtk/gtklistbox.c
@@ -2832,17 +2832,20 @@ gtk_list_box_move_cursor (GtkListBox      *box,
               GSequenceIter *cursor_iter;
               GSequenceIter *next_iter;
 
-              /* A NULL row should only happen when the list box didn't
-               * have enough rows to fill its height and the user made
-               * a page movement down, so the count must be positive */
-              g_assert (count > 0);
-
-              cursor_iter = ROW_PRIV (priv->cursor_row)->iter;
-              next_iter = gtk_list_box_get_last_visible (box, cursor_iter);
-
-              if (next_iter)
+              if (count > 0)
+                {
+                  cursor_iter = ROW_PRIV (priv->cursor_row)->iter;
+                  next_iter = gtk_list_box_get_last_visible (box, cursor_iter);
+
+                  if (next_iter)
+                    {
+                      row = g_sequence_get (next_iter);
+                      end_y = ROW_PRIV (row)->y;
+                    }
+                }
+              else
                 {
-                  row = g_sequence_get (next_iter);
+                  row = gtk_list_box_get_row_at_index (box, 0);
                   end_y = ROW_PRIV (row)->y;
                 }
             }