Fix unchecked addition.

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -425,7 +425,7 @@ pub(super) fn construct_trampoline_onion_packet(
 		.map(|p| {
 			let mut payload_len = LengthCalculatingWriter(0);
 			p.write(&mut payload_len).expect("Failed to calculate length");
-			payload_len.0 + 32
+			payload_len.0.checked_add(32).expect("Excessive payload size")
 		})
 		.sum();