Use correct commitment number/point in initial commitment_signed

When splicing a channel, the initial commitment_signed should use the
same commitment number and point previously sent. Account for this by
adjusting these to use the previous commitment number and point, since
the next expected one is stored.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -5583,10 +5583,19 @@ where
 		SP::Target: SignerProvider,
 		L::Target: Logger,
 	{
+		let mut commitment_number = self.cur_counterparty_commitment_transaction_number;
+		let mut commitment_point = self.counterparty_cur_commitment_point.unwrap();
+
+		// Use the previous commitment number and point when splicing since they shouldn't change.
+		if commitment_number != INITIAL_COMMITMENT_NUMBER {
+			commitment_number += 1;
+			commitment_point = self.counterparty_prev_commitment_point.unwrap();
+		}
+
 		let commitment_data = self.build_commitment_transaction(
 			funding,
-			self.cur_counterparty_commitment_transaction_number,
-			&self.counterparty_cur_commitment_point.unwrap(),
+			commitment_number,
+			&commitment_point,
 			false,
 			false,
 			logger,
@@ -7019,8 +7028,8 @@ where
 			.context
 			.build_commitment_transaction(
 				pending_splice_funding,
-				self.context.cur_counterparty_commitment_transaction_number,
-				&self.context.counterparty_cur_commitment_point.unwrap(),
+				self.context.cur_counterparty_commitment_transaction_number + 1,
+				&self.context.counterparty_prev_commitment_point.unwrap(),
 				false,
 				false,
 				logger,