Do not fassert that object returned from InvalidatingLRUCache is valid

The InvalidatingLRUCache contains valid objects. However, once an object has been returned, it can be invalidated at any point. This means that it is unsafe to fassert that it is valid. In the past, fassert 17003 was evaluated inside of the mutex which excluded invalidation. It should now be either removed, or moved to a place where the condition is maintained.