19863 fix wedge

The NullPointerExceptions caused by attempting to send messages during disconnect can leave scalang in a state where it is always trying to monitor non-live process (having failed to send a MonitorExitMessage).

This patchset clarifieds when a null channel should not occur and gracefully handles a null channel when disconnecting. It also closes a tiny race condition in connectRequested for a more idiomatic (and atomic) use of compareAndSet.

BugzID: 19863