[gnome-control-center] shell: prevent a warning if the monitor number is not available yet

[Thomas Wood <thos src gnome org>]

commit eca503bcc5a4a8ee8662c68d1f4c0daec9924c72
Author: Thomas Wood <thomas wood intel com>
Date:   Fri Aug 2 13:27:09 2013 +0100

    shell: prevent a warning if the monitor number is not available yet
    
    https://bugzilla.gnome.org/show_bug.cgi?id=705297

 shell/cc-window.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/shell/cc-window.c b/shell/cc-window.c
index 5118b22..0c500f2 100644
--- a/shell/cc-window.c
+++ b/shell/cc-window.c
@@ -1258,6 +1258,9 @@ get_monitor_height (CcWindow *self)
   GdkScreen *screen;
   GdkRectangle rect;
 
+  if (self->priv->monitor_num < 0)
+    return 0;
+
   /* We cannot use workarea here, as this wouldn't
    * be updated when we read it after a monitors-changed signal */
   screen = gtk_widget_get_screen (GTK_WIDGET (self));