[gtk/wip/albfan/protect-begin_move_drag] Check begin_move_drag is defined

[Alberto Fanjul <albfan src gnome org>]

commit 99ae64ccce4fa316fd6d52ea69c0ec05ed5bbaac
Author: Alberto Fanjul <albfan gnome org>
Date:   Sat Mar 21 09:35:17 2020 +0100

    Check begin_move_drag is defined
    
    GdkOffScreenWindow do not define begin_move_drag, so ensure it is defined before call it

 gdk/gdkwindow.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 6ae0f86710..46e57437fb 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -11244,8 +11244,9 @@ gdk_window_begin_move_drag_for_device (GdkWindow *window,
                                        gint       root_y,
                                        guint32    timestamp)
 {
-  GDK_WINDOW_IMPL_GET_CLASS (window->impl)->begin_move_drag (window,
-                                                             device, button, root_x, root_y, timestamp);
+  GdkWindowImplClass *win_impl_class = GDK_WINDOW_IMPL_GET_CLASS (window->impl);
+  if (win_impl_class->begin_move_drag)
+    win_impl_class->begin_move_drag (window, device, button, root_x, root_y, timestamp);
 }
 
 /**