fix Include the weight of the old funding input for splice fee estimate

Author: optout21

lightning/src/ln/channel.rs

@@ -4706,11 +4706,13 @@ fn check_v2_funding_inputs_sufficient(
 	is_splice: bool, funding_feerate_sat_per_1000_weight: u32,
 ) -> Result<u64, ChannelError> {
 	let mut total_input_witness_weight = Weight::from_wu(funding_inputs.iter().map(|(_, _, w)| w.to_wu()).sum());
+	let mut funding_inputs_len = funding_inputs.len();
 	if is_initiator && is_splice {
-		// consider the weight of the witness needed for spending the old funding transaction
+		// consider the weight of the input and witness needed for spending the old funding transaction
+		funding_inputs_len += 1;
 		total_input_witness_weight += Weight::from_wu(FUNDING_TRANSACTION_WITNESS_WEIGHT);
 	}
-	let estimated_fee = estimate_v2_funding_transaction_fee(is_initiator, funding_inputs.len(), total_input_witness_weight, funding_feerate_sat_per_1000_weight);
+	let estimated_fee = estimate_v2_funding_transaction_fee(is_initiator, funding_inputs_len, total_input_witness_weight, funding_feerate_sat_per_1000_weight);
 
 	let mut total_input_sats = 0u64;
 	for (idx, input) in funding_inputs.iter().enumerate() {