[gtk/gtk-3-24] gtkstack: fix null pointer dereference

[Daniel Boles <dboles src gnome org>]

commit adbaee796de3ccb9b0a95b298fb0bfd00c2fcdd4
Author: Hugo Lefeuvre <hle debian org>
Date:   Wed Sep 26 16:59:59 2018 -0400

    gtkstack: fix null pointer dereference
    
    The gtk_stack_snapshot_slide() function dereferences the
    last_visible_child pointer without proper != NULL ckeck. This might
    result in NULL pointer dereference and crash if last_visible_child is
    invalid.
    
    Add a != NULL check before dereferencing the pointer.
    
    cherry-picked from https://gitlab.gnome.org/GNOME/gtk/merge_requests/361

 gtk/gtkstack.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)
---
diff --git a/gtk/gtkstack.c b/gtk/gtkstack.c
index 6815488a3e..51a3a732e5 100644
--- a/gtk/gtkstack.c
+++ b/gtk/gtkstack.c
@@ -2088,12 +2088,14 @@ gtk_stack_draw_slide (GtkWidget *widget,
       x += priv->last_visible_surface_allocation.x;
       y += priv->last_visible_surface_allocation.y;
 
-
-      if (gtk_widget_get_valign (priv->last_visible_child->widget) == GTK_ALIGN_END &&
-          priv->last_visible_widget_height > allocation.height)
-        y -= priv->last_visible_widget_height - allocation.height;
-      else if (gtk_widget_get_valign (priv->last_visible_child->widget) == GTK_ALIGN_CENTER)
-        y -= (priv->last_visible_widget_height - allocation.height) / 2;
+      if (priv->last_visible_child != NULL)
+        {
+          if (gtk_widget_get_valign (priv->last_visible_child->widget) == GTK_ALIGN_END &&
+              priv->last_visible_widget_height > allocation.height)
+            y -= priv->last_visible_widget_height - allocation.height;
+          else if (gtk_widget_get_valign (priv->last_visible_child->widget) == GTK_ALIGN_CENTER)
+            y -= (priv->last_visible_widget_height - allocation.height) / 2;
+        }
 
       cairo_save (cr);
       cairo_set_source_surface (cr, priv->last_visible_surface, x, y);