bgpd: make bgp_nlri_parse_encap conform with other nlri_parse funcs

* bgp_encap.{c,h} (bgp_nlri_parse_encap) afi is already in the NLRI argument.
  update or withdraw is signalled by attr being non-NULL or NULL.

* bgp_packet.c: (update_receive) fixup to match, and also make the attr
  argument conform with NLRI_ATTR_ARG for correct error handling on
  optional, transitive, partial, attributes.
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index 26e4d8f..b8a38fa 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -1943,12 +1943,12 @@
       if (mp_update.length
 	  && mp_update.afi == AFI_IP
 	  && mp_update.safi == SAFI_ENCAP)
-	bgp_nlri_parse_encap (mp_update.afi, peer, &attr, &mp_update, 0);
+	bgp_nlri_parse_encap (peer, NLRI_ATTR_ARG, &mp_update);
 
       if (mp_withdraw.length
 	  && mp_withdraw.afi == AFI_IP
 	  && mp_withdraw.safi == SAFI_ENCAP)
-	bgp_nlri_parse_encap (mp_withdraw.afi, peer, &attr, &mp_withdraw, 1);
+	bgp_nlri_parse_encap (peer, NULL, &mp_withdraw);
 
       if (! withdraw_len
 	  && mp_withdraw.afi == AFI_IP
@@ -1967,12 +1967,12 @@
       if (mp_update.length 
 	  && mp_update.afi == AFI_IP6 
 	  && mp_update.safi == SAFI_ENCAP)
-	bgp_nlri_parse_encap (mp_update.afi, peer, &attr, &mp_update, 0);
+	bgp_nlri_parse_encap (peer, NLRI_ATTR_ARG, &mp_update);
 
       if (mp_withdraw.length 
 	  && mp_withdraw.afi == AFI_IP6
 	  && mp_withdraw.safi == SAFI_ENCAP)
-	bgp_nlri_parse_encap (mp_withdraw.afi, peer, &attr, &mp_withdraw, 1);
+	bgp_nlri_parse_encap (peer, NULL, &mp_withdraw);
 
       if (! withdraw_len
 	  && mp_withdraw.afi == AFI_IP6