Fix fuzzer-found underflow

Author: valentinewallace

lightning/src/onion_message/packet.rs

@@ -69,7 +69,7 @@ impl LengthReadable for Packet {
 		let public_key = Readable::read(r)?;
 
 		let mut hop_data = Vec::new();
-		let hop_data_len = r.total_bytes() as usize - 66; // 1 (version) + 33 (pubkey) + 32 (HMAC) = 66
+		let hop_data_len = r.total_bytes().saturating_sub(66) as usize; // 1 (version) + 33 (pubkey) + 32 (HMAC) = 66
 		let mut read_idx = 0;
 		while read_idx < hop_data_len {
 			let mut read_buffer = [0; READ_BUFFER_SIZE];