Enforce the use of accessors on JValue so we can ensure sign extension.

Change-Id: I34810af7661d8674c84707afe5cfdf9e2c233f8b
diff --git a/src/object.cc b/src/object.cc
index f863a39..158f0e6 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -570,7 +570,7 @@
                               PrettyMethod(this).c_str(), GetCode(), stub,
                               Runtime::Current()->IsStarted() ? "true" : "false");
     if (result != NULL) {
-      result->j = 0;
+      result->SetJ(0);
     }
   }