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_encap.c b/bgpd/bgp_encap.c
index d0beb1b..1a09ba6 100644
--- a/bgpd/bgp_encap.c
+++ b/bgpd/bgp_encap.c
@@ -126,14 +126,13 @@
 
 int
 bgp_nlri_parse_encap(
-    afi_t		afi,
     struct peer		*peer,
     struct attr		*attr, 		/* Need even for withdraw */
-    struct bgp_nlri	*packet,
-    int			withdraw)	/* 0=update, !0 = withdraw */
+    struct bgp_nlri	*packet)
 {
   u_char *pnt;
   u_char *lim;
+  afi_t afi = packet->afi;
   struct prefix p;
   int psize = 0;
   int prefixlen;
@@ -227,7 +226,7 @@
 	    inet_ntop (p.family, &p.u.prefix, buf, BUFSIZ),
 	    p.prefixlen);
 
-      if (!withdraw) {
+      if (attr) {
 	bgp_update (peer, &p, attr, afi, SAFI_ENCAP,
 		    ZEBRA_ROUTE_BGP, BGP_ROUTE_NORMAL, &prd, NULL, 0);
       } else {