Fix unsafe unwraps while getting fail data

Also refactored this section in
`get_htlc_failure_reason_from_failure_code` to use nested if let
instead of checking .is_none and unwrapping because I thought it
made the control flow clearer.

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -3525,25 +3525,29 @@ where
 			FailureCode::TemporaryNodeFailure => HTLCFailReason::from_failure_code(*failure_code as u16),
 			FailureCode::RequiredNodeFeatureMissing => HTLCFailReason::from_failure_code(*failure_code as u16),
 			FailureCode::ExpiryTooSoon => {
+				let mut expiry_fail_reason = None;
 				let short_channel_id = htlc.prev_hop.short_channel_id;
-				let (counterparty_node_id, chan_id) = self.short_to_chan_info.read().unwrap().get(&short_channel_id).cloned().unwrap();
-				let per_peer_state = self.per_peer_state.read().unwrap();
-				let peer_state = per_peer_state.get(&counterparty_node_id);
-				if peer_state.is_none() {
-					return self.get_htlc_fail_reason_from_failure_code(&FailureCode::IncorrectOrUnknownPaymentDetails, htlc)
+				let cp_node_chan_id = self.short_to_chan_info.read().unwrap().get(&short_channel_id).cloned();
+				if let Some((counterparty_node_id, chan_id)) = cp_node_chan_id {
+					let per_peer_state = self.per_peer_state.read().unwrap();
+					if let Some(peer_state) = per_peer_state.get(&counterparty_node_id) {
+						let peer_state = peer_state.lock().unwrap();
+						if let Some(chan) = peer_state.channel_by_id.get(&chan_id) {
+							if let Ok(channel_update) = self.get_channel_update_for_onion(short_channel_id, chan) {
+								let mut channel_update_data = Vec::new();
+								(channel_update.serialized_length() as u16 + 2).write(&mut channel_update_data).expect("Writes cannot fail");
+								msgs::ChannelUpdate::TYPE.write(&mut channel_update_data).expect("Writes cannot fail");
+								channel_update.write(&mut channel_update_data).expect("Writes cannot fail");
+								expiry_fail_reason = Some(HTLCFailReason::reason(*failure_code as u16, channel_update_data));
+							}
+						}
+					}
 				}
-				let peer_state = peer_state.unwrap().lock().unwrap();
-				let chan = peer_state.channel_by_id.get(&chan_id);
-				if chan.is_none() {
-					return self.get_htlc_fail_reason_from_failure_code(&FailureCode::IncorrectOrUnknownPaymentDetails, htlc)
+				if let Some(expiry_fail_reason) = expiry_fail_reason {
+					expiry_fail_reason
+				} else {
+					self.get_htlc_fail_reason_from_failure_code(&FailureCode::IncorrectOrUnknownPaymentDetails, htlc)
 				}
-				let chan = chan.unwrap();
-				let channel_update = self.get_channel_update_for_onion(short_channel_id, chan).unwrap();
-				let mut channel_update_data = Vec::new();
-				(channel_update.serialized_length() as u16 + 2).write(&mut channel_update_data).expect("Writes cannot fail");
-				msgs::ChannelUpdate::TYPE.write(&mut channel_update_data).expect("Writes cannot fail");
-				channel_update.write(&mut channel_update_data).expect("Writes cannot fail");
-				HTLCFailReason::reason(*failure_code as u16, channel_update_data)
 			},
 			FailureCode::IncorrectOrUnknownPaymentDetails => {
 				let mut htlc_msat_height_data = htlc.value.to_be_bytes().to_vec();