fixup: Improve prediction of commitment stats in can_accept_incoming_htlc

Turn the unwrap_or(0) into an expect; `can_accept_incoming_htlc` is
called when the inbound HTLC is already irrevocably committed to the
channel, so we should have rejected the HTLC at `update_add_htlc` if it
exhausts the counterparty's balance.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -4586,7 +4586,9 @@ where
 			if !funding.get_channel_type().supports_anchors_zero_fee_htlc_tx() {
 				remote_fee_incl_fee_spike_buffer_htlc_msat *= FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE;
 			}
-			let remote_balance_before_fee_msat = next_remote_commitment_stats.counterparty_balance_msat.unwrap_or(0);
+			// We unwrap here; if the HTLC exhausts the counterparty's balance, we should have rejected it at `update_add_htlc`, here the HTLC is already
+			// irrevocably committed to the channel.
+			let remote_balance_before_fee_msat = next_remote_commitment_stats.counterparty_balance_msat.expect("The counterparty's balance before fees should never underflow");
 			if remote_balance_before_fee_msat.saturating_sub(funding.holder_selected_channel_reserve_satoshis * 1000) < remote_fee_incl_fee_spike_buffer_htlc_msat {
 				log_info!(logger, "Attempting to fail HTLC due to fee spike buffer violation in channel {}. Rebalancing is required.", &self.channel_id());
 				return Err(LocalHTLCFailureReason::FeeSpikeBuffer);