lightningd: Restore forwarding of legacy onions.

Partial revert of 43a833e405
"lightningd: remove support for legacy onion format."; we restore the
ability to decode legacy onions for forwarding, but not to generate them.
(We don't accept them properly since making payment_secret compulsory
anyway, so no real change there!)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Removed: Protocol: ... but we still forward legacy HTLC onions for now.

common/sphinx.c

@@ -627,7 +627,7 @@ struct route_step *process_onionpacket(
 	payload_size = onion_payload_length(paddedheader,
 					    tal_bytelen(msg->routinginfo),
 					    has_realm,
-					    &valid);
+					    &valid, NULL);
 
 	/* Can't decode?  Treat it as terminal. */
 	if (!valid) {