lnrpc/walletrpc: reject PSBT packets w/o any UTXO input info

Fixes https://github.com/lightningnetwork/lnd/issues/6567

docs/release-notes/release-notes-0.15.0.md

@@ -183,6 +183,8 @@ from occurring that would result in an erroneous force close.](https://github.co
 
 * [Fixes an issue related to HTLCs on lease enforced channels that can lead to itest flakes](https://github.com/lightningnetwork/lnd/pull/6605/files)
 
+* [Fixes a bug that would cause `SignPsbt` to panic w/ an underspecified packet](https://github.com/lightningnetwork/lnd/pull/6611)
+
 ## Routing
 
 * [Add a new `time_pref` parameter to the QueryRoutes and SendPayment APIs](https://github.com/lightningnetwork/lnd/pull/6024) that

lnrpc/walletrpc/walletkit_server.go

@@ -1234,6 +1234,19 @@ func (w *WalletKit) SignPsbt(_ context.Context, req *SignPsbtRequest) (
 		return nil, fmt.Errorf("error parsing PSBT: %v", err)
 	}
 
+	// Before we attempt to sign the packet, ensure that every input either
+	// has a witness UTXO, or a non witness UTXO.
+	for idx := range packet.UnsignedTx.TxIn {
+		in := packet.Inputs[idx]
+
+		// Doesn't have either a witness or non witness UTXO so we need
+		// to exit here as otherwise signing will fail.
+		if in.WitnessUtxo == nil && in.NonWitnessUtxo == nil {
+			return nil, fmt.Errorf("input (index=%v) doesn't "+
+				"specify any UTXO info", idx)
+		}
+	}
+
 	// Let the wallet do the heavy lifting. This will sign all inputs that
 	// we have the UTXO for. If some inputs can't be signed and don't have
 	// witness data attached, they will just be skipped.