fix initialization of ReetrancyGuard storage variable

contracts/mocks/ReentrancyMock.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../utils/ReentrancyGuard.sol";
 import "./ReentrancyAttack.sol";
 
 
-contract ReentrancyMock is ReentrancyGuard {
+contract ReentrancyMock is Initializable, ReentrancyGuard {
 
   uint256 public counter;
 
   constructor() public {
+    ReentrancyGuard.initialize();
     counter = 0;
   }
 

contracts/utils/ReentrancyGuard.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 
 /**
  * @title Helps contracts guard against reentrancy attacks.
@@ -7,10 +8,14 @@ pragma solidity ^0.4.24;
  * @dev If you mark a function `nonReentrant`, you should also
  * mark it `external`.
  */
-contract ReentrancyGuard {
+contract ReentrancyGuard is Initializable {
 
   /// @dev counter to allow mutex lock with only one SSTORE operation
-  uint256 private _guardCounter = 1;
+  uint256 private _guardCounter;
+
+  function initialize() public initializer {
+    _guardCounter = 1;
+  }
 
   /**
    * @dev Prevents a contract from calling itself, directly or indirectly.