Reduces mul execution cost

[0xtotem]

This is a minor PR that reduces mul execution cost from 21769 down to 21748 when a != 0.

This PR recommends to add the a == 0 test on top of the function but it turns out that with Solidity ^0.6.12 this isn't true.

Code use for testing:

pragma solidity ^0.6.12;

contract SafeMath {
    function mul(uint256 a, uint256 b) public pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    function mul2(uint256 a, uint256 b) public pure returns (uint256) {
        uint256 c = a * b;
        require(a == 0 || c / a == b, "SafeMath: multiplication overflow");

        return c;
    }
}