lnrpc+itest: fix panic with missing UTXO info

Fixes #6396.
This commit fixes a panic that occurred when trying to sign for a
Taproot output without specifying the full UTXO information for each
input. Instead of panicking an error is now returned.

lnrpc/signrpc/signer_server.go

@@ -376,6 +376,24 @@ func (s *Server) SignOutputRaw(_ context.Context, in *SignReq) (*SignResp,
 			InputIndex:        int(signDesc.InputIndex),
 			PrevOutputFetcher: prevOutputFetcher,
 		})
+
+		// Are we trying to sign for a Taproot output? Then we need all
+		// previous outputs being declared, otherwise we'd run into a
+		// panic later on.
+		if txscript.IsPayToTaproot(signDesc.Output.PkScript) {
+			for idx, txIn := range txToSign.TxIn {
+				utxo := prevOutputFetcher.FetchPrevOutput(
+					txIn.PreviousOutPoint,
+				)
+				if utxo == nil {
+					return nil, fmt.Errorf("error signing "+
+						"taproot output, transaction "+
+						"input %d is missing its "+
+						"previous outpoint information",
+						idx)
+				}
+			}
+		}
 	}
 
 	// Now that we've mapped all the proper sign descriptors, we can

lntest/itest/lnd_taproot_test.go

@@ -232,6 +232,29 @@ func testTaprootScriptSpend(ctxt context.Context, t *harnessTest,
 		PkScript: p2trPkScript,
 		Value:    800_000,
 	}}
+
+	// Before we actually sign, we want to make sure that we get an error
+	// when we try to sign for a Taproot output without specifying all UTXO
+	// information.
+	_, err = net.Alice.SignerClient.SignOutputRaw(
+		ctxt, &signrpc.SignReq{
+			RawTxBytes: buf.Bytes(),
+			SignDescs: []*signrpc.SignDescriptor{{
+				Output:        utxoInfo[0],
+				InputIndex:    0,
+				KeyDesc:       keyDesc,
+				Sighash:       uint32(txscript.SigHashDefault),
+				WitnessScript: leaf2.Script,
+			}},
+		},
+	)
+	require.Error(t.t, err)
+	require.Contains(
+		t.t, err.Error(), "error signing taproot output, transaction "+
+			"input 0 is missing its previous outpoint information",
+	)
+
+	// Do the actual signing now.
 	signResp, err := net.Alice.SignerClient.SignOutputRaw(
 		ctxt, &signrpc.SignReq{
 			RawTxBytes: buf.Bytes(),