[mutter/wayland] main: Kill a runtime warning

[Jasper St. Pierre <jstpierre src gnome org>]

commit 24c5290d7f03992b113581c114dae859ba916c1b
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Tue Feb 18 20:30:36 2014 -0500

    main: Kill a runtime warning
    
    g_setenv doesn't take NULL for a value.

 src/core/main.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/core/main.c b/src/core/main.c
index c4bdeee..84da691 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -343,7 +343,8 @@ meta_select_display (char *display_arg)
   else
     display_name = g_getenv ("MUTTER_DISPLAY");
 
-  g_setenv ("DISPLAY", display_name, TRUE);
+  if (display_name)
+    g_setenv ("DISPLAY", display_name, TRUE);
 }
 
 static void