Can't flush bad naming stub if no java.naming.provider.url specified

The NamingContext class maintains a static cache of server-side naming stubs, keyed by the address:port of the server. Entries from this cache are meant to be removed if an IOException is caught when invoking on the stub, thus ensuring that problem stubs are discarded and a new stub lookup is forced.

This cache clearing process breaks if java.naming.provider.url is not set. The clearing algorithm is in NamingContext.removeServer(Hashtable) is a no-op if the naming environment does not include java.naming.provider.url. Based on a comment on the method, the reasoning behind this is to avoid flushing the "local" (i.e. in-vm) server from the cache. But, the way the naming acquisition process works, a direct ref to the in-vm server will never be stored in the cache anyway, so this distinction in not valid.