ospf6d: fix crash if zebra_interface_state_read can't find interface

* ospf6_zebra.c: (ospf6_zebra_if_state_update) zebra_interface_state_read
  may return NULL, if it can't find an interface, deal with it.
diff --git a/ospf6d/ospf6_zebra.c b/ospf6d/ospf6_zebra.c
index 0a8ac3e..881771a 100644
--- a/ospf6d/ospf6_zebra.c
+++ b/ospf6d/ospf6_zebra.c
@@ -132,6 +132,9 @@
   struct interface *ifp;
 
   ifp = zebra_interface_state_read (zclient->ibuf);
+  if (ifp == NULL)
+    return 0;
+  
   if (IS_OSPF6_DEBUG_ZEBRA (RECV))
     zlog_debug ("Zebra Interface state change: "
                 "%s index %d flags %llx metric %d mtu %d",