fixup! multi: decode zero-length onion message payloads

Author: gijswijs

payload.go

@@ -101,18 +101,22 @@ func (hp *HopPayload) Decode(r io.Reader) error {
 
 	var payloadSize uint16
 
-	// If the HopPayload isn't guaranteed to be a TLV payload, we check the
-	// first byte to see if it is a legacy payload.
-	if hp.Type != PayloadTLV && isLegacyPayloadByte(peekByte[0]) {
+	// If the HopPayload might be a legacy payload (indicated by the Type
+	// being equal to the zero-value PayloadLegacy), we check the first
+	// byte to see if it is a legacy payload.
+	if hp.Type == PayloadLegacy && isLegacyPayloadByte(peekByte[0]) {
 		payloadSize = legacyPayloadSize()
 	} else {
-		// If the first byte doesn't indicate a legacy payload, then it
-		// *must* be a TLV payload.
+		// Either this is already known to be a TLV payload, or the
+		// first byte indicates that this is a TLV payload (i.e., the
+		// first byte is not 0x00).
 		payloadSize, err = tlvPayloadSize(bufReader)
 		if err != nil {
 			return err
 		}
 
+		// We still need to set the payload type in case it was the
+		// zero-value, PayloadLegacy, AND the first byte was not 0x00.
 		hp.Type = PayloadTLV
 	}