window-actor: Guard against %NULL frame mask

Creating a new cogl texture may fail, in which case the intent to
free it will crash. While something is clearly wrong (insanely
large window, oom, ...), crashing the WM is harsh and we should
try to avoid it if at all possible, so carry on.

https://bugzilla.gnome.org/show_bug.cgi?id=722266

src/compositor/meta-window-actor.c

@@ -1714,6 +1714,7 @@ build_and_scan_frame_mask (MetaWindowActor       *self,
     }
 
   meta_shaped_texture_set_mask_texture (stex, mask_texture);
+  if (mask_texture)
     cogl_object_unref (mask_texture);
 
   g_free (mask_data);