x/evm/keeper: CheckSenderBalance should ensure that txData.Cost() is ALWAYS positive firstly before checking for sender account sufficiency

[odeke-em]

This code here https://github.com/tharsis/ethermint/blob/b8ae5984c54f79d76cfc95518f77b4226a05b940/x/evm/keeper/utils.go#L83-L94

test that the keeper has sufficient balance to pay for the cost of a transaction.

However, notice that it does a big.Int comparison https://github.com/tharsis/ethermint/blob/b8ae5984c54f79d76cfc95518f77b4226a05b940/x/evm/keeper/utils.go#L86

Essentially if the account balance ever gets negative, there could be a scenario in which I could craft TxData to have a "negative" cost where it is being used as a big.Int which show it as negative but perhaps much later on, that cost could get cast to a uint* which will then wrap around to a positive value.

Proposal

For safety and consistency, let's ensure that we reject negative costs with an explicit check

if cost.Sign() < 0 {
      // Report the error here about the cost being negative.
      return stacktrace.Propagate(
			sdkerrors.Wrapf(
				sdkerrors.ErrInsufficientFunds, // Or whatever error it is.
				"tx cost (%s%s) is negative and invalid",  txData.Cost(), denom,
			),
			...
		)
}

// Same for checking if the balance is >= 0
balanceBigInt := balance.Amount.BigInt()
if balanceBigInt.Sign() < 0 {
     // Report that the balance has dipped less than 0.
     return stacktrace.Propagate(
            ....
    }
}

// Now finally do the prior comparisons
if balance.Amount.BigInt().Cmp(cost) < 0 {
    return ...
}