Remove previous holder HTLC data on splice locked when necessary

If while a splice is pending, the channel happens to not have any
commitment updates, but did prior to the splice being negotiated, it's
possible that we end up with bogus holder HTLC data for the previous
commitment. After the splice becomes locked, we've successfully
transitioned to the new funding transaction, but that funding
transaction never had a commitment transaction negotiated for the
previous state.

Backport of c418034

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -4175,6 +4175,14 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			self.funding.prev_holder_commitment_tx.clone(),
 		);
 
+		// It's possible that no commitment updates happened during the lifecycle of the pending
+		// splice's `FundingScope` that was promoted. If so, our `prev_holder_htlc_data` is
+		// now irrelevant, since there's no valid previous commitment that exists for the current
+		// funding transaction that could be broadcast.
+		if self.funding.prev_holder_commitment_tx.is_none() {
+			self.prev_holder_htlc_data.take();
+		}
+
 		let no_further_updates_allowed = self.no_further_updates_allowed();
 
 		// The swap above places the previous `FundingScope` into `pending_funding`.