lightningd: fix inconsistency without COMPAT enabled.

We don't expect payment or payment->route_channels to be NULL without an
old db, but putting an assert there reveals that we try to fail an HTLC
which has already succeeded in 'test_onchain_unwatch'.

Obviously we only want to fail an HTLC which goes onchain if we don't
already have the preimage!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/pay.c

@@ -482,6 +482,9 @@ void payment_failed(struct lightningd *ld, const struct htlc_out *hout,
 					  PAYMENT_FAILED, NULL);
 		return;
 	}
+#else
+	assert(payment);
+	assert(payment->route_channels);
 #endif
 
 	/* This gives more details than a generic failure message */

lightningd/peer_htlcs.c

@@ -860,8 +860,8 @@ void onchain_failed_our_htlc(const struct channel *channel,
 	struct lightningd *ld = channel->peer->ld;
 	struct htlc_out *hout = find_htlc_out_by_ripemd(channel, &htlc->ripemd);
 
-	/* Don't fail twice! */
-	if (hout->failuremsg || hout->failcode)
+	/* Don't fail twice (or if already succeeded)! */
+	if (hout->failuremsg || hout->failcode || hout->preimage)
 		return;
 
 	hout->failcode = WIRE_PERMANENT_CHANNEL_FAILURE;