G1 Service thread register_task can be used after shutdown

There is a race when shutting down the VM, where it is possible to add new tasks to the service thread after it has been shut down. This is not used today, but the upcoming work for concurrent uncommit might trigger this situation.

The problem is that the tasks already in the task_queue are allocated on stack, and after shutdown those are no longer available. So a new addition will look at stale tasks.