Bugfix when the parent bundle is missing.

When I had added this feature I verified that it would be OK to look up
a non-existant bundle, and even added a little note in the code.
But, it's the ResourceBundle that will be null, a MessageBundle
is always returned. And so, when it tried to initialize that bogus
MessageBundle and recursed into this same patch of code, the newly
added check for __parent would cause an NPE.