Implement _calculateRewardAmount more efficiently

[code423n4]

Handle

0x0x0x

Vulnerability details

https://github.com/pooltogether/v4-periphery/blob/b520faea26bcf60371012f6cb246aa149abd3c7d/contracts/TwabRewards.sol#L302-L321 is as follows:

        uint256 _averageBalance = _ticket.getAverageBalanceBetween(
            _user,
            uint64(_epochStartTimestamp),
            uint64(_epochEndTimestamp)
        );

        uint64[] memory _epochStartTimestamps = new uint64[](1);
        _epochStartTimestamps[0] = uint64(_epochStartTimestamp);

        uint64[] memory _epochEndTimestamps = new uint64[](1);
        _epochEndTimestamps[0] = uint64(_epochEndTimestamp);

        uint256[] memory _averageTotalSupplies = _ticket.getAverageTotalSuppliesBetween(
            _epochStartTimestamps,
            _epochEndTimestamps
        );

        if (_averageTotalSupplies[0] > 0) {
            return (_promotion.tokensPerEpoch * _averageBalance) / _averageTotalSupplies[0];
        }

        return 0;
    }

Since _averageBalance is always bigger than _averageTotalSupplies[0]. We can implement the, if statement earlier. This will ensure to output 0 earlier. Furthermore, _averageBalance is in stack and this check costs less gas. Therefore, the code can be implemented as follows:

        uint256 _averageBalance = _ticket.getAverageBalanceBetween(
            _user,
            uint64(_epochStartTimestamp),
            uint64(_epochEndTimestamp)
        );
                if (_averageBalance > 0) {
                uint64[] memory _epochStartTimestamps = new uint64[](1);
                _epochStartTimestamps[0] = uint64(_epochStartTimestamp);

                uint64[] memory _epochEndTimestamps = new uint64[](1);
                _epochEndTimestamps[0] = uint64(_epochEndTimestamp);

                uint256[] memory _averageTotalSupplies = _ticket.getAverageTotalSuppliesBetween(
                    _epochStartTimestamps,
                    _epochEndTimestamps
                );

            return (_promotion.tokensPerEpoch * _averageBalance) / _averageTotalSupplies[0];
        }

        return 0;
    }