bgpd: tests - add null pointer protection to fix bgp test failures

Signed-off-by: Lou Berger <lberger@labn.net>
diff --git a/bgpd/bgp_nexthop.c b/bgpd/bgp_nexthop.c
index d1c5c86..bb07eac 100644
--- a/bgpd/bgp_nexthop.c
+++ b/bgpd/bgp_nexthop.c
@@ -594,6 +594,9 @@
 void
 bgp_address_destroy (void)
 {
+  if (bgp_address_hash == NULL)
+    return;
+
   hash_clean(bgp_address_hash, NULL);
   hash_free(bgp_address_hash);
   bgp_address_hash = NULL;