privacy: Don't put an extra separator before the first row

The separator function is called for each row, to allow creating
the separator that goes before that row. We don't want a separator
before the first row, so we return NULL if before == NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=689746

panels/privacy/cc-privacy-panel.c

@@ -616,6 +616,9 @@ update_separator_func (GtkWidget **separator,
                        GtkWidget  *before,
                        gpointer    user_data)
 {
+  if (before == NULL)
+    return;
+
   if (*separator == NULL)
     {
       *separator = gtk_separator_new (GTK_ORIENTATION_HORIZONTAL);