bgpd: fix memory leak for extra attributes

this fixes commit b881c7074bb698aeb1b099175b325734fc6e44d2
diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index 29877eb..4e604b1 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -675,6 +675,7 @@
     }
 
   bgp_attr_unintern_sub (&tmp);
+  bgp_attr_extra_free (&tmp);
 }
 
 void