Clarified the rationaly behind the mul branch test. (#970)

contracts/math/SafeMath.sol

@@ -11,9 +11,13 @@ library SafeMath {
   * @dev Multiplies two numbers, throws on overflow.
   */
   function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
+    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
+    // benefit is lost if 'b' is also tested.
+    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
     if (a == 0) {
       return 0;
     }
+
     c = a * b;
     assert(c / a == b);
     return c;