htlc: Return an invalid payload error if payload is invalid

We somehow were always returning `badonion` when really it is an invalid
payload inside the onion.

lightningd/peer_htlcs.c

@@ -981,9 +981,12 @@ htlc_accepted_hook_callback(struct htlc_accepted_hook_payload *request,
 	case htlc_accepted_continue:
 		/* *Now* we barf if it failed to decode */
 		if (!request->payload) {
-			log_debug(channel->log, "Failing HTLC because of an invalid payload");
-			local_fail_in_htlc_badonion(hin,
-						    WIRE_INVALID_ONION_PAYLOAD);
+			log_debug(channel->log,
+				  "Failing HTLC because of an invalid payload");
+			local_fail_in_htlc(hin,
+					   take(towire_invalid_onion_payload(
+					       NULL, request->failtlvtype,
+					       request->failtlvpos)));
 		} else if (rs->nextcase == ONION_FORWARD) {
 			forward_htlc(hin, hin->cltv_expiry,
 				     request->payload->amt_to_forward,

tests/test_pay.py

@@ -2806,7 +2806,6 @@ caused a crash in 0.8.0, so we then disallowed it.
     assert inv['amount_received_msat'] == Millisatoshi(1001)
 
 
-@pytest.mark.xfail(strict=True)
 def test_reject_invalid_payload(node_factory):
     """Send an onion payload with an unknown even type.