[metacity] window: fix crash if workspace is null

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 24d35569bdb78d1da3b53ed1a6d81d365d60bed0
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Tue Feb 28 15:27:59 2017 +0200

    window: fix crash if workspace is null

 src/core/window.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/core/window.c b/src/core/window.c
index 3a582b2..5ba5182 100644
--- a/src/core/window.c
+++ b/src/core/window.c
@@ -2129,9 +2129,10 @@ windows_overlap (const MetaWindow *w1, const MetaWindow *w2)
 static gboolean
 window_would_be_covered (const MetaWindow *newbie)
 {
-  MetaWorkspace *workspace = newbie->workspace;
+  MetaWorkspace *workspace;
   GList *tmp, *windows;
 
+  workspace = meta_window_get_workspace ((MetaWindow *) newbie);
   windows = meta_workspace_list_windows (workspace);
 
   tmp = windows;