solidity漏洞实践（2）

    pragma solidity ^0.6.12;
    pragma experimental ABIEncoderV2;
    library SafeMath {
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
    
        return c;
    }
    
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        uint256 c = a - b;
    
        return c;
    }
    
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {