bgpd: improve cleanup in bgp_delete()

Signed-off-by: Lou Berger <lberger@labn.net>
diff --git a/bgpd/bgp_zebra.c b/bgpd/bgp_zebra.c
index 4a25aa9..1be4440 100644
--- a/bgpd/bgp_zebra.c
+++ b/bgpd/bgp_zebra.c
@@ -1081,3 +1081,13 @@
 
   bgp_nexthop_buf = stream_new(BGP_NEXTHOP_BUF_SIZE);
 }
+
+void
+bgp_zebra_destroy(void)
+{
+  if (zclient == NULL)
+    return;
+  zclient_stop(zclient);
+  zclient_free(zclient);
+  zclient = NULL;
+}