[metacity] prefs: check if we have a valid display

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 6ec606c2a1b8ab4fcc691adaf852c76f63611af0
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Fri Jun 12 22:20:03 2015 +0300

    prefs: check if we have a valid display
    
    Display will be null when libmetacity-private will be used in
    compiz.

 src/core/prefs.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/core/prefs.c b/src/core/prefs.c
index f404aff..26e26f7 100644
--- a/src/core/prefs.c
+++ b/src/core/prefs.c
@@ -1920,6 +1920,9 @@ meta_prefs_get_compositing_manager (void)
 
   display = meta_get_display ();
 
+  if (!display)
+    return compositing_manager;
+
   if (!compositing_manager)
     warned = FALSE;