conf: handle null pointer in virNetDevVlanFormat

Other *Format() functions (e.g. virNetDevBandwidthFormat()) return
with no action when called with a NULL *Def pointer. This makes
virNetDevVlanFormat() consistent with that behavior.

src/conf/netdev_vlan_conf.c

 /*
- * Copyright (C) 2009-2013 Red Hat, Inc.
+ * Copyright (C) 2009-2014 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -144,7 +144,7 @@ virNetDevVlanFormat(const virNetDevVlan *def, virBufferPtr buf)
 {
     size_t i;
 
-    if (def->nTags == 0)
+    if (!(def && def->nTags))
         return 0;
 
     if (!def->tag) {