htlcswitch: split parsing and validation of TLV payloads

When handling blinded errors, we need to know whether there was a
blinding key in our payload when we successfully parsed our payload
but then found an invalid set of fields. The combination of
parsing and validation in NewPayloadFromReader means that we don't know
whether a blinding point was available to us by the time the error is
returned.

This commit splits parsing and validation into two functions so that
we can take a look at what we actually pulled of the payload in between
parsing and TLV validation.

htlcswitch/hop/iterator.go

@@ -112,14 +112,20 @@ func (r *sphinxHopIterator) HopPayload() (*Payload, error) {
 	// to decode only what we need to make routing decisions.
 	case sphinx.PayloadTLV:
 		isFinal := r.processedPacket.Action == sphinx.ExitNode
-		payload, parsed, err := NewPayloadFromReader(
+		payload, parsed, err := ParseTLVPayload(
 			bytes.NewReader(r.processedPacket.Payload.Payload),
-			isFinal, r.blindingKit.UpdateAddBlinding.IsSome(),
 		)
 		if err != nil {
 			return nil, err
 		}
 
+		if err := ValidateTLVPayload(
+			parsed, isFinal,
+			r.blindingKit.UpdateAddBlinding.IsSome(),
+		); err != nil {
+			return nil, err
+		}
+
 		// If we had an encrypted data payload present, pull out our
 		// forwarding info from the blob.
 		if payload.encryptedData != nil {