[zebra] Fix crash in test_zebra

* kernel_null.c: we're pretending to add an address, so
  set IFC_REAL. If this isn't done, calling into the 'got address
  from kernel' half of zebra will implicitly-withdraw the ifc, which
  could cause a crash in test_zebra.c.
diff --git a/zebra/kernel_null.c b/zebra/kernel_null.c
index 94b7b3c..6b96c6d 100644
--- a/zebra/kernel_null.c
+++ b/zebra/kernel_null.c
@@ -22,6 +22,7 @@
 int kernel_address_add_ipv4 (struct interface *a, struct connected *b)
 {
   zlog_debug ("%s", __func__);
+  SET_FLAG (b->conf, ZEBRA_IFC_REAL);
   connected_add_ipv4 (a, 0, &b->address->u.prefix4, b->address->prefixlen, 
                       (b->destination ? &b->destination->u.prefix4 : NULL), 
                       NULL);