tapgarden: ensure change output index is set after anchor funding

Add a check to verify that the change output index is defined after
anchor funding. Since the funding request guarantees a change output,
the index should be known.

Modify the `MockWalletAnchor.FundPsbt` method to compute a reasonable
change output index.

Author: ffranr

tapgarden/mock.go

@@ -182,6 +182,10 @@ func (m *MockWalletAnchor) FundPsbt(_ context.Context, packet *psbt.Packet,
 	packet.UnsignedTx.AddTxOut(&changeOutput)
 	packet.Outputs = append(packet.Outputs, psbt.POutput{})
 
+	// The change output was added last, so it will be the last output in
+	// the list. Update the change index to reflect this.
+	changeIdx = int32(len(packet.Outputs) - 1)
+
 	// We always have the change output be the second output, so this means
 	// the Taproot Asset commitment will live in the first output.
 	pkt := &tapsend.FundedPsbt{

tapgarden/planter.go

@@ -749,6 +749,12 @@ func (c *ChainPlanter) fundGenesisPsbt(ctx context.Context,
 		return nil, fmt.Errorf("unable to fund psbt: %w", err)
 	}
 
+	// Sanity check the funded PSBT.
+	if fundedGenesisPkt.ChangeOutputIndex == -1 {
+		return nil, fmt.Errorf("undefined change output index in " +
+			"funded anchor transaction")
+	}
+
 	log.Infof("Funded GenesisPacket for batch: %x", batchKey)
 	log.Tracef("GenesisPacket: %v", spew.Sdump(fundedGenesisPkt))