ln: set zero second stage fees for zero fee commitments

Update second_stage_tx_fees_sat to return zero for zero fee commitments.
As is the case for anchors_zero_fee_commitments, this changes ensures
that we won't trim the second stage transaction fee off of the HTLC
amount.

Author: carlaKC

lightning/src/ln/channel.rs

@@ -138,7 +138,9 @@ enum FeeUpdateState {
 pub(super) fn second_stage_tx_fees_sat(
 	channel_type: &ChannelTypeFeatures, feerate_sat_per_1000_weight: u32,
 ) -> (u64, u64) {
-	if channel_type.supports_anchors_zero_fee_htlc_tx() {
+	if channel_type.supports_anchors_zero_fee_htlc_tx()
+		|| channel_type.supports_anchor_zero_fee_commitments()
+	{
 		(0, 0)
 	} else {
 		(