[audit]#8 handle sqrt coner case

contracts/lib/SafeMath.sol

@@ -53,7 +53,7 @@ library SafeMath {
     }
 
     function sqrt(uint256 x) internal pure returns (uint256 y) {
-        uint256 z = (x + 1) / 2;
+        uint256 z = add(x, 1) / 2;
         y = x;
         while (z < y) {
             y = z;