withdraw: check error from RegisterSpendNtfn before using channels

handleWithdrawal did not check the error returned by
RegisterSpendNtfn before spawning a goroutine to read from the
notification channels. If registration failed, spentChan would be nil
and the goroutine would block forever on a nil channel read.

Add the missing error check so the function returns early on
registration failure.

Author: hieblmi

staticaddr/withdraw/manager.go

@@ -676,6 +676,9 @@ func (m *Manager) handleWithdrawal(ctx context.Context,
 		ctx, &d.OutPoint, addrParams.PkScript,
 		int32(d.ConfirmationHeight),
 	)
+	if err != nil {
+		return fmt.Errorf("unable to register spend ntfn: %w", err)
+	}
 
 	go func() {
 		select {