Check if a batch is expected for commitment_signed

When receiving a commitment_signed message, if there are any pending
splices then we are expected to receive the message as a part of a
batch. Otherwise, the spec dictates that we should send an error and
fail the channel.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -5975,6 +5975,10 @@ impl<SP: Deref> FundedChannel<SP> where
 	{
 		self.commitment_signed_check_state()?;
 
+		if self.pending_splice.is_some() {
+			return Err(ChannelError::close("Got a single commitment_signed message when expecting a batch".to_owned()));
+		}
+
 		let updates = self
 			.context
 			.validate_commitment_signed(&self.funding, &self.holder_commitment_point, msg, logger)