prepare previousModel before preConditions to prevent mutation

Author: nanov

lib/definitions/aggregate.js

@@ -738,13 +738,13 @@ _.extend(Aggregate.prototype, {
       throw new Error('You are not allowed to set a value in this step!');
     };
 
+    var previousModel = new AggregateModel(aggregateModel.id, aggregateModel);
+
     this.checkPreConditions(cmd, aggregateModel, function (err) {
       if (err) {
         return callback(err);
       }
 
-      var previousModel = new AggregateModel(aggregateModel.id, aggregateModel);
-
       // attach apply function
       aggregateModel.apply = applyHelper(self, aggregateModel, cmd);