Merge pull request #28 from SkaterDad/Amazon-Fix-NPE

Fix crashes when using Amazon provider

extension-iap/src/java/com/defold/iap/IapAmazon.java

@@ -45,6 +45,7 @@ public class IapAmazon implements PurchasingListener {
         this.activity = activity;
         this.autoFinishTransactions = autoFinishTransactions;
         this.listProductsListeners = new HashMap<RequestId, IListProductsListener>();
+        this.listProductsCommandPtrs = new HashMap<RequestId, Long>();
         this.purchaseListeners = new HashMap<RequestId, IPurchaseListener>();
         PurchasingService.registerListener(activity, this);
     }
@@ -96,6 +97,10 @@ public class IapAmazon implements PurchasingListener {
         }
         PurchasingService.notifyFulfillment(receipt, FulfillmentResult.FULFILLED);
     }
+    
+    public void acknowledgeTransaction(final String purchaseToken, final IPurchaseListener purchaseListener) {
+        // Stub to prevent errors.
+    }
 
     private void doGetPurchaseUpdates(final IPurchaseListener listener, final boolean reset) {
         synchronized (purchaseListeners) {