bgpd: fix memory leak on malformed attribute

When bgp_attr_parse returns BGP_ATTR_PARSE_ERROR, it may already have
parsed and allocated some attributes before hitting that error.  Free
the attr's data before returning.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index 80651f1..65c6cac 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -1720,7 +1720,10 @@
       attr_parse_ret = bgp_attr_parse (peer, &attr, attribute_len, 
 			    &mp_update, &mp_withdraw);
       if (attr_parse_ret == BGP_ATTR_PARSE_ERROR)
-	return -1;
+	{
+	  bgp_attr_unintern_sub (&attr);
+	  return -1;
+	}
     }
   
   /* Logging the attribute. */