[gtk/gtk-3-24] x11: Add necessary NULL check

[Carlos Garnacho <carlosg src gnome org>]

commit 0a37b564d46d5e0cd5f1e4e7877ab8caf270e3de
Author: Benjamin Otte <otte redhat com>
Date:   Sat Dec 16 02:24:12 2017 +0100

    x11: Add necessary NULL check
    
    (Backported from master 3438dcdd4e0)

 gdk/x11/gdkdevicemanager-xi2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gdk/x11/gdkdevicemanager-xi2.c b/gdk/x11/gdkdevicemanager-xi2.c
index 20cb4f4cbd..ad17ff4a8c 100644
--- a/gdk/x11/gdkdevicemanager-xi2.c
+++ b/gdk/x11/gdkdevicemanager-xi2.c
@@ -1027,7 +1027,8 @@ handle_property_change (GdkX11DeviceManagerXI2 *device_manager,
   device = g_hash_table_lookup (device_manager->id_table,
                                 GUINT_TO_POINTER (ev->deviceid));
 
-  if (ev->property == gdk_x11_get_xatom_by_name ("Wacom Serial IDs"))
+  if (device != NULL &&
+      ev->property == gdk_x11_get_xatom_by_name ("Wacom Serial IDs"))
     {
       GdkDeviceTool *tool = NULL;
       guint serial_id = 0, tool_id = 0;