fix(multiple-arbitrable-transaction): fix re-entrancy

fix by changing statement order, calling external contract before require checks

contracts/standard/arbitration/MultipleArbitrableTransaction.sol

@@ -201,10 +201,11 @@ contract MultipleArbitrableTransaction {
      */
     function payArbitrationFeeByBuyer(uint _transactionID) public payable {
         Transaction storage transaction = transactions[_transactionID];
+        uint arbitrationCost = arbitrator.arbitrationCost(arbitratorExtraData);
+
         require(transaction.status < Status.DisputeCreated, "Dispute has already been created or because the transaction has been executed.");
         require(msg.sender == transaction.buyer, "The caller must be the buyer.");
 
-        uint arbitrationCost = arbitrator.arbitrationCost(arbitratorExtraData);
         transaction.buyerFee += msg.value;
         // Require that the total paid to be at least the arbitration cost.
         require(transaction.buyerFee >= arbitrationCost, "The buyer fee must cover arbitration costs.");
@@ -226,10 +227,11 @@ contract MultipleArbitrableTransaction {
      */
     function payArbitrationFeeBySeller(uint _transactionID) public payable {
         Transaction storage transaction = transactions[_transactionID];
+        uint arbitrationCost = arbitrator.arbitrationCost(arbitratorExtraData);
+
         require(transaction.status < Status.DisputeCreated, "Dispute has already been created or because the transaction has been executed.");
         require(msg.sender == transaction.seller, "The caller must be the seller.");
 
-        uint arbitrationCost = arbitrator.arbitrationCost(arbitratorExtraData);
         transaction.sellerFee += msg.value;
         // Require that the total pay at least the arbitration cost.
         require(transaction.sellerFee >= arbitrationCost, "The seller fee must cover arbitration costs.");