walletrpc: disallow locking with internal ID

The internal lock ID that the wallet kit subserver uses to lock inputs
for itself shouldn't be allowed to be used when locking inputs manually
over the RPC.

lnrpc/walletrpc/walletkit_server.go

@@ -329,6 +329,12 @@ func (w *WalletKit) LeaseOutput(ctx context.Context,
 		return nil, errors.New("id must be 32 random bytes")
 	}
 
+	// Don't allow our internal ID to be used externally for locking. Only
+	// unlocking is allowed.
+	if lockID == LndInternalLockID {
+		return nil, errors.New("reserved id cannot be used")
+	}
+
 	op, err := unmarshallOutPoint(req.Outpoint)
 	if err != nil {
 		return nil, err