account may have currency with zero balance

[zoro]

For the account:
https://dex-atlantic.binance.org/api/v1/account/bnb1xwalxpaes9r0z0fqdy70j3kz6aayetegur38gl

MDAB-D42, BOLT-4C6, AWC-986 get zero balance, while the account have hold BHFT-BBE before but did not show it.

The reason is that AWC-986 is sold by order, while BHFT-BBE is transferred to others.
refer: https://explorer.binance.org/api/v1/txs?address=bnb1xwalxpaes9r0z0fqdy70j3kz6aayetegur38gl&txAsset=AWC-986&page=1&rows=100

When handle orders, tran.unlock - tran.out is equal to zero and cause the problem.

func (kp *Keeper) doTransfer(ctx sdk.Context, tran *Transfer) sdk.Error {
	account := kp.am.GetAccount(ctx, tran.accAddress).(types.NamedAccount)
	newLocked := account.GetLockedCoins().Minus(sdk.Coins{sdk.NewCoin(tran.outAsset, tran.unlock)})
	// these two non-negative check are to ensure the Transfer gen result is correct before we actually operate the acc.
	// they should never happen, there would be a severe bug if happen and we have to cancel all orders when app restarts.
	if !newLocked.IsNotNegative() {
		panic(fmt.Errorf(
			"no enough locked tokens to unlock, oid: %s, newLocked: %s, unlock: %d",
			tran.Oid,
			newLocked.String(),
			tran.unlock))
	}
	if tran.unlock < tran.out {
		panic(errors.New("unlocked tokens cannot cover the expense"))
	}
	account.SetLockedCoins(newLocked)
	account.SetCoins(account.GetCoins().
		Plus(sdk.Coins{sdk.NewCoin(tran.inAsset, tran.in)}).
		Plus(sdk.Coins{sdk.NewCoin(tran.outAsset, tran.unlock-tran.out)}))

	kp.am.SetAccount(ctx, account)
	kp.logger.Debug("Performed Trade Allocation", "account", account, "allocation", tran.String())
	return nil
}