bgpd: Squash spurious "unknown afi" log messages

* bgp_packet.c: (bgp_update_receive) doesn't differentiate between NLRIs that
  are 0 AFI/SAFI cause they weren't set, and those because a peer sent a
  bogus AFI/SAFI, before sending sending what may be a misleading, spurious
  log message.  Check the .nlri pointer is set and avoid this.

Incorporating a suggestion from: G. Paul Ziemba <unp@ziemba.us>
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index 740b0f1..9743d06 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -1645,6 +1645,7 @@
   memset (&attr, 0, sizeof (struct attr));
   memset (&extra, 0, sizeof (struct attr_extra));
   memset (&nlris, 0, sizeof nlris);
+
   attr.extra = &extra;
 
   s = peer->ibuf;
@@ -1781,6 +1782,8 @@
   /* Parse any given NLRIs */
   for (i = NLRI_UPDATE; i < NLRI_TYPE_MAX; i++)
     {
+      if (!nlris[i].nlri) continue;
+      
       /* We use afi and safi as indices into tables and what not.  It would
        * be impossible, at this time, to support unknown afi/safis.  And
        * anyway, the peer needs to be configured to enable the afi/safi