Offers with currency must set amount. (#3140)

Adds check and corresponding test from lightning/bolts#1279

Author: thomash-acinq

eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/OfferTypes.scala

@@ -309,6 +309,7 @@ object OfferTypes {
     def validate(records: TlvStream[OfferTlv]): Either[InvalidTlvPayload, Offer] = {
       if (records.get[OfferDescription].isEmpty && records.get[OfferAmount].nonEmpty) return Left(MissingRequiredTlv(UInt64(10)))
       if (records.get[OfferNodeId].isEmpty && records.get[OfferPaths].forall(_.paths.isEmpty)) return Left(MissingRequiredTlv(UInt64(22)))
+      if (records.get[OfferCurrency].nonEmpty && records.get[OfferAmount].isEmpty) return Left(MissingRequiredTlv(UInt64(8)))
       if (records.unknown.exists(!isOfferTlv(_))) return Left(ForbiddenTlv(records.unknown.find(!isOfferTlv(_)).get.tag))
       Right(Offer(records))
     }

eclair-core/src/test/resources/offers-test.json

@@ -578,6 +578,11 @@
     "valid": false,
     "bolt12": "lno1pqpzwyqkyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
   },
+  {
+    "description": "Missing offer_amount with offer_currency",
+    "valid": false,
+    "bolt12": "lno1qcp4256ypgx9getnwss8vetrw3hhyuckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
   {
     "description": "Missing offer_issuer_id and no offer_path",
     "valid": false,