tapsend: error out instead of panicking

If there is no root locator defined when creating a split commitment, we
return an error instead of running into a panic later on.

Author: guggero

tapsend/send.go

@@ -124,6 +124,12 @@ var (
 	// in a virtual transaction that was already committed to an anchor
 	// output.
 	ErrAssetNotSigned = errors.New("asset not signed")
+
+	// ErrNoRootLocator is an error that is returned when a split commitment
+	// is created without a split root output.
+	ErrNoRootLocator = errors.New(
+		"cannot create split commitment without split root output",
+	)
 )
 
 var (
@@ -590,6 +596,13 @@ func PrepareOutputAssets(ctx context.Context, vPkt *tappsbt.VPacket) error {
 		splitLocators = append(splitLocators, &locator)
 	}
 
+	// If we're creating a split commitment, there must be a split root.
+	// If none of the outputs was designated as the split root, then
+	// something was done incorrectly.
+	if rootLocator == nil {
+		return ErrNoRootLocator
+	}
+
 	splitCommitment, err := commitment.NewSplitCommitment(
 		ctx, splitCommitmentInputs, rootLocator, splitLocators...,
 	)

tapsend/send_test.go

@@ -777,6 +777,21 @@ var prepareOutputAssetsTestCases = []testCase{{
 		return nil
 	},
 	err: nil,
+}, {
+	name: "asset split with missing root locator",
+	f: func(t *testing.T) error {
+		state := initSpendScenario(t)
+
+		pkt := createPacket(
+			state.address1, state.asset2PrevID,
+			state, state.asset2InputAssets, false,
+		)
+
+		pkt.Outputs[0].Type = tappsbt.TypeSimple
+
+		return tapsend.PrepareOutputAssets(context.Background(), pkt)
+	},
+	err: tapsend.ErrNoRootLocator,
 }, {
 	name: "full value non-interactive send with un-spendable change",
 	f: func(t *testing.T) error {