`CDP.sol#update.sol` can be optimized

[code423n4]

Handle

0x0x0x

Vulnerability details

Concept

The current code is:

  function update(Data storage _self, Context storage _ctx) internal {
    uint256 _earnedYield = _self.getEarnedYield(_ctx);
    if (_earnedYield > _self.totalDebt) {
      uint256 _currentTotalDebt = _self.totalDebt;
      _self.totalDebt = 0;
      _self.totalCredit = _earnedYield.sub(_currentTotalDebt);
    } else {
      _self.totalDebt = _self.totalDebt.sub(_earnedYield);
    }
    _self.lastAccumulatedYieldWeight = _ctx.accumulatedYieldWeight;
  }

We cache _self.totalDebt, but it is not required, since we can use it before we change it. This code block can be replaced with:

  function update(Data storage _self, Context storage _ctx) internal {
    uint256 _earnedYield = _self.getEarnedYield(_ctx);
    if (_earnedYield > _self.totalDebt) {
      _self.totalCredit = _earnedYield.sub(_self.totalDebt);
      _self.totalDebt = 0;
    } else {
      _self.totalDebt = _self.totalDebt.sub(_earnedYield);
    }
    _self.lastAccumulatedYieldWeight = _ctx.accumulatedYieldWeight;
  }

By doing so, we don't cache _self.totalDebt just to use it once.