fix: use saturating sub while calculating change amount (#4026)

Co-authored-by: Daniel <daniel@chainflip.io>
Co-authored-by: dandanlen <3168260+dandanlen@users.noreply.github.com>

state-chain/pallets/cf-environment/src/lib.rs

@@ -426,14 +426,19 @@ impl<T: Config> Pallet<T> {
 		match utxo_selection_type {
 			UtxoSelectionType::SelectAllForRotation => {
 				let available_utxos = BitcoinAvailableUtxos::<T>::take();
-				(!available_utxos.is_empty()).then_some(available_utxos).map(|available_utxos| {
-					(
-						available_utxos.clone(),
-						available_utxos.iter().map(|Utxo { amount, .. }| *amount).sum::<u64>() -
-							(available_utxos.len() as u64) * fee_per_input_utxo -
-							fee_per_output_utxo - min_fee_required_per_tx,
-					)
-				})
+				(!available_utxos.is_empty()).then_some(available_utxos).and_then(
+					|available_utxos| {
+						available_utxos
+							.iter()
+							.map(|Utxo { amount, .. }| *amount)
+							.sum::<u64>()
+							.checked_sub(
+								((available_utxos.len() as u64) * fee_per_input_utxo) +
+									fee_per_output_utxo + min_fee_required_per_tx,
+							)
+							.map(|change_amount| (available_utxos, change_amount))
+					},
+				)
 			},
 			UtxoSelectionType::Some { output_amount, number_of_outputs } =>
 				BitcoinAvailableUtxos::<T>::try_mutate(|available_utxos| {