[mutter] xwayland: Not all xwayland surface have a window

[Robert Mader <rmader src gnome org>]

commit b8355a66860027a7265efed9945c2af522b65749
Author: Olivier Fourdan <ofourdan redhat com>
Date:   Fri Feb 21 11:57:29 2020 +0100

    xwayland: Not all xwayland surface have a window
    
    `meta_xwayland_surface_get_relative_coordinates()` may cause a crash if
    the Xwayland surface has no window associated.
    
    That can be observed when using drag and drop from an X11 window to a
    Wayland native window:
    
    ```
        at src/core/window.c:4503
        at src/wayland/meta-xwayland-surface.c:200
        at src/wayland/meta-wayland-surface.c:1517
        at src/wayland/meta-wayland-pointer.c:1048
        at src/wayland/meta-wayland-pointer.c:840
        at src/wayland/meta-wayland-pointer.c:865
        at src/wayland/meta-wayland-pointer.c:954
        at src/wayland/meta-wayland-pointer.c:456
        at src/wayland/meta-wayland-pointer.c:993
        at src/wayland/meta-wayland-data-device.c:1004
        at src/wayland/meta-wayland-data-device.c:1278
        at src/wayland/meta-xwayland-dnd.c:326
    ```
    
    Check if the xwayland surface has an associated MetaWindow prior to get
    its buffer rect.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/1073

 src/wayland/meta-xwayland-surface.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/wayland/meta-xwayland-surface.c b/src/wayland/meta-xwayland-surface.c
index 9aef3c27d..3233dcc26 100644
--- a/src/wayland/meta-xwayland-surface.c
+++ b/src/wayland/meta-xwayland-surface.c
@@ -195,9 +195,11 @@ meta_xwayland_surface_get_relative_coordinates (MetaWaylandSurfaceRole *surface_
                                                 float                  *out_sy)
 {
   MetaXwaylandSurface *xwayland_surface = META_XWAYLAND_SURFACE (surface_role);
-  MetaRectangle window_rect;
+  MetaRectangle window_rect = { 0 };
+
+  if (xwayland_surface->window)
+    meta_window_get_buffer_rect (xwayland_surface->window, &window_rect);
 
-  meta_window_get_buffer_rect (xwayland_surface->window, &window_rect);
   *out_sx = abs_x - window_rect.x;
   *out_sy = abs_y - window_rect.y;
 }