Do some validation in splice_ack before taking a &mut self

As much as possible, we want to only mutate state once we are done with
input validation.

It also helps with introducing helper validation functions that take a
`&self` in the next commits.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -10831,26 +10831,17 @@ where
 		ES::Target: EntropySource,
 		L::Target: Logger,
 	{
-		let pending_splice = if let Some(ref mut pending_splice) = &mut self.pending_splice {
-			pending_splice
-		} else {
-			return Err(ChannelError::Ignore(format!("Channel is not in pending splice")));
-		};
-
 		// TODO(splicing): Add check that we are the splice (quiescence) initiator
 
-		let funding_negotiation_context = match pending_splice.funding_negotiation.take() {
+		let funding_negotiation_context = match &self
+			.pending_splice
+			.as_ref()
+			.ok_or(ChannelError::Ignore(format!("Channel is not in pending splice")))?
+			.funding_negotiation
+		{
 			Some(FundingNegotiation::AwaitingAck(context)) => context,
-			Some(FundingNegotiation::ConstructingTransaction(funding, constructor)) => {
-				pending_splice.funding_negotiation =
-					Some(FundingNegotiation::ConstructingTransaction(funding, constructor));
-				return Err(ChannelError::WarnAndDisconnect(format!(
-					"Got unexpected splice_ack; splice negotiation already in progress"
-				)));
-			},
-			Some(FundingNegotiation::AwaitingSignatures(funding)) => {
-				pending_splice.funding_negotiation =
-					Some(FundingNegotiation::AwaitingSignatures(funding));
+			Some(FundingNegotiation::ConstructingTransaction(_, _))
+			| Some(FundingNegotiation::AwaitingSignatures(_)) => {
 				return Err(ChannelError::WarnAndDisconnect(format!(
 					"Got unexpected splice_ack; splice negotiation already in progress"
 				)));
@@ -10885,6 +10876,17 @@ where
 			self.funding.get_value_satoshis(),
 		);
 
+		let pending_splice =
+			self.pending_splice.as_mut().expect("We should have returned an error earlier!");
+		// TODO: Good candidate for a let else statement once MSRV >= 1.65
+		let funding_negotiation_context = if let Some(FundingNegotiation::AwaitingAck(context)) =
+			pending_splice.funding_negotiation.take()
+		{
+			context
+		} else {
+			panic!("We should have returned an error earlier!");
+		};
+
 		let mut interactive_tx_constructor = funding_negotiation_context
 			.into_interactive_tx_constructor(
 				&self.context,