An integer underflow has been found in the latest version of ZCFees

An integer underflow has been found in the latest version of ZCFees. The variables 'currPeriodIdx' and 'lastPeriodExecIdx' are both unsigned integers, and the result of the minus operation may be a negative integer which leads to an underflow. The attackers can modify the current timestamp of the transaction somehow and block the execution of the process function.