tapfreighter: don't attempt to send burns with courier

We forgot to exclude burn outputs when it comes to burns, which then
resulted in the `invalid proof courier URI address: parse "": empty url`
error, failing to confirm burns.
So the transfer was pending forever.

Author: guggero

tapfreighter/chain_porter.go

@@ -606,6 +606,8 @@ func (p *ChainPorter) transferReceiverProof(pkg *sendPackage) error {
 			return nil
 		}
 
+		// Un-spendable means this is a tombstone output resulting from
+		// a split.
 		unSpendable, err := out.ScriptKey.IsUnSpendable()
 		if err != nil {
 			return fmt.Errorf("error checking if script key is "+
@@ -618,6 +620,17 @@ func (p *ChainPorter) transferReceiverProof(pkg *sendPackage) error {
 			return nil
 		}
 
+		// Burns are also always kept local and not sent to any
+		// receiver.
+		if len(out.WitnessData) > 0 && asset.IsBurnKey(
+			out.ScriptKey.PubKey, out.WitnessData[0],
+		) {
+
+			log.Debugf("Not transferring proof for burn script "+
+				"key %x", key.SerializeCompressed())
+			return nil
+		}
+
 		// We just look for the full proof in the list of final proofs
 		// by matching the content of the proof suffix.
 		var receiverProof *proof.AnnotatedProof