Check result handler is not null

If caller passes a null result handler to the proxy methods, we need
to check it when getting the result. Otherwise we try to invoke a method
on a null object

Change-Id: Ic2f456c36cb542b782b6a6d0d4d607ccf0987496

Author: Manuel Roman

tools/aidl/generate_java_rpc.cpp

@@ -784,7 +784,10 @@ generate_result_dispatcher_method(const method_type* method,
     }
 
     // Call the callback method
-    dispatchMethod->statements->Add(realCall);
+    IfStatement* ifst = new IfStatement;
+        ifst->expression = new Comparison(new FieldVariable(THIS_VALUE, "callback"), "!=", NULL_VALUE);
+    dispatchMethod->statements->Add(ifst);
+    ifst->statements->Add(realCall);
 }
 
 static void