ln: return early and assert in fee calculation and checks

When we have zero fee commitments, we don't need to calculate our fee
rate or check that it isn't stale because it is always zero.

Co-authored-by: Matt Corallo <git@bluematt.me>

Author: carlaKC

lightning/src/ln/channel.rs

@@ -4966,6 +4966,11 @@ where
 		let context = &self;
 		assert!(funding.is_outbound());
 
+		if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+			debug_assert_eq!(context.feerate_per_kw, 0);
+			return 0;
+		}
+
 		let (htlc_success_tx_fee_sat, htlc_timeout_tx_fee_sat) = second_stage_tx_fees_sat(
 			funding.get_channel_type(), context.feerate_per_kw,
 		);
@@ -5067,6 +5072,11 @@ where
 	fn next_remote_commit_tx_fee_msat(
 		&self, funding: &FundingScope, htlc: Option<HTLCCandidate>, fee_spike_buffer_htlc: Option<()>,
 	) -> u64 {
+		if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+			debug_assert_eq!(self.feerate_per_kw, 0);
+			return 0
+		}
+
 		debug_assert!(htlc.is_some() || fee_spike_buffer_htlc.is_some(), "At least one of the options must be set");
 
 		let context = &self;
@@ -7414,6 +7424,12 @@ where
 			// unable to increase the fee, we don't try to force-close directly here.
 			return Ok(());
 		}
+
+		if self.funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+			debug_assert_eq!(self.context.feerate_per_kw, 0);
+			return Ok(());
+		}
+
 		if self.context.feerate_per_kw < min_feerate {
 			log_info!(logger,
 				"Closing channel as feerate of {} is below required {} (the minimum required rate over the past day)",