lnrpc/walletrpc: use current height as height hint for cpfp

In this commit, we address an issue that would cause us to scan from the
genesis block for the spend of an output that we wish to use to raise
the fee of a transaction through CPFP. This was due to setting a 0
height hint when constructing the input required by the sweeper and was
discovered due to the recently added validation checks at the chain
notifier level. We'll now use the current height as the height hint
instead as the sweeper will end up creating a new transaction that
spends the input.

lnrpc/walletrpc/driver.go

@@ -51,6 +51,14 @@ func createNewSubServer(configRegistry lnrpc.SubServerConfigDispatcher) (lnrpc.S
 	case config.KeyRing == nil:
 		return nil, nil, fmt.Errorf("KeyRing must be set to create " +
 			"WalletKit RPC server")
+
+	case config.Sweeper == nil:
+		return nil, nil, fmt.Errorf("Sweeper must be set to create " +
+			"WalletKit RPC server")
+
+	case config.Chain == nil:
+		return nil, nil, fmt.Errorf("Chain must be set to create " +
+			"WalletKit RPC server")
 	}
 
 	return New(config)