math overflow fix

contracts/lib/SafeMath.sol

@@ -32,6 +32,11 @@ library SafeMath {
         return a / b;
     }
 
+    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
+        require(b > 0, "MOD_ERROR");
+        return a % b;
+    }
+
     function divCeil(uint256 a, uint256 b) internal pure returns (uint256) {
         uint256 quotient = div(a, b);
         uint256 remainder = a - quotient * b;