Switch error hop iteration to Peekable

We currently check whether our hop is the last in the path by accessing
the hops vector by the next index. However, once we start handling
Trampoline hops that will become inadequate. Instead, we switch it to
check whether there is a subsequent element in the iterator.

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -988,7 +988,8 @@ where
 	.expect("Route we used spontaneously grew invalid keys in the middle of it?");
 
 	// Handle packed channel/node updates for passing back for the route handler
-	for (route_hop_idx, (route_hop_option, shared_secret)) in onion_keys.into_iter().enumerate() {
+	let mut iterator = onion_keys.into_iter().peekable();
+	while let Some((route_hop_option, shared_secret)) = iterator.next() {
 		let route_hop = match route_hop_option.as_ref() {
 			Some(hop) => hop,
 			None => {
@@ -1008,13 +1009,16 @@ where
 		// The failing hop includes either the inbound channel to the recipient or the outbound channel
 		// from the current hop (i.e., the next hop's inbound channel).
 		// For 1-hop blinded paths, the final `path.hops` entry is the recipient.
-		is_from_final_node = route_hop_idx + 1 == path.hops.len() && num_blinded_hops <= 1;
+		// In our case that means that if we're on the last iteration, and there is no more than one
+		// blinded hop, the current iteration references the last non-blinded hop.
+		let next_hop = iterator.peek();
+		is_from_final_node = next_hop.is_none() && num_blinded_hops <= 1;
 		let failing_route_hop = if is_from_final_node {
 			route_hop
 		} else {
-			match path.hops.get(route_hop_idx + 1) {
-				Some(hop) => hop,
-				None => {
+			match next_hop {
+				Some((Some(hop), _)) => hop,
+				_ => {
 					// The failing hop is within a multi-hop blinded path.
 					#[cfg(not(test))]
 					{