wayland: skip pointer and keyboard events without a surface

Pointer and keyboard events can be received after the surface has been
destroyed, in which case the surface will be NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=693338

gdk/wayland/gdkdevice-wayland.c

@@ -550,6 +550,9 @@ pointer_handle_enter (void              *data,
   GdkWaylandDisplay *wayland_display =
     GDK_WAYLAND_DISPLAY (device->display);
 
+  if (!surface)
+    return;
+
   _gdk_wayland_display_update_serial (wayland_display, serial);
 
   device->pointer_focus = wl_surface_get_user_data(surface);
@@ -589,6 +592,9 @@ pointer_handle_leave (void              *data,
   GdkWaylandDisplay *wayland_display =
     GDK_WAYLAND_DISPLAY (device->display);
 
+  if (!surface)
+    return;
+
   _gdk_wayland_display_update_serial (wayland_display, serial);
 
   event = gdk_event_new (GDK_LEAVE_NOTIFY);
@@ -777,6 +783,9 @@ keyboard_handle_enter (void               *data,
   GdkWaylandDisplay *wayland_display =
     GDK_WAYLAND_DISPLAY (device->display);
 
+  if (!surface)
+    return;
+
   _gdk_wayland_display_update_serial (wayland_display, serial);
 
   device->keyboard_focus = wl_surface_get_user_data(surface);
@@ -808,6 +817,9 @@ keyboard_handle_leave (void               *data,
   GdkWaylandDisplay *wayland_display =
     GDK_WAYLAND_DISPLAY (device->display);
 
+  if (!surface)
+    return;
+
   _gdk_wayland_display_update_serial (wayland_display, serial);
 
   _gdk_wayland_window_remove_focus (device->keyboard_focus);