Fix blinded path serialization in Route

`Route`'s blinded_path serialization logic writes a blinded path
`Option` per path hop, however on read we (correctly) only read one
blinded path `Option` per path. This causes serialization of
`Route`s with blinded paths to fail to round-trip.

Here we fix this by writing blinded paths per path.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -508,20 +508,20 @@ impl Writeable for Route {
 		write_ver_prefix!(writer, SERIALIZATION_VERSION, MIN_SERIALIZATION_VERSION);
 		(self.paths.len() as u64).write(writer)?;
 		let mut blinded_tails = Vec::new();
-		for path in self.paths.iter() {
+		for (idx, path) in self.paths.iter().enumerate() {
 			(path.hops.len() as u8).write(writer)?;
-			for (idx, hop) in path.hops.iter().enumerate() {
+			for hop in path.hops.iter() {
 				hop.write(writer)?;
-				if let Some(blinded_tail) = &path.blinded_tail {
-					if blinded_tails.is_empty() {
-						blinded_tails = Vec::with_capacity(path.hops.len());
-						for _ in 0..idx {
-							blinded_tails.push(None);
-						}
-					}
-					blinded_tails.push(Some(blinded_tail));
-				} else if !blinded_tails.is_empty() { blinded_tails.push(None); }
 			}
+			if let Some(blinded_tail) = &path.blinded_tail {
+				if blinded_tails.is_empty() {
+					blinded_tails = Vec::with_capacity(path.hops.len());
+					for _ in 0..idx {
+						blinded_tails.push(None);
+					}
+				}
+				blinded_tails.push(Some(blinded_tail));
+			} else if !blinded_tails.is_empty() { blinded_tails.push(None); }
 		}
 		write_tlv_fields!(writer, {
 			// For compatibility with LDK versions prior to 0.0.117, we take the individual