If we use getcurrentFiberPtr() then we will possibly run in a assert
witin that function.

It's 'ns' stupid.

and use that hook in Actor.

This fixes a race condition in the actor loop between 'queue.get()'
and 'setState(Running)' racing against waitUntilIdle(long)'s check if

queue.size() == 0 && state == Retrieving

Because right after the get() operation is finished, the queue size is
0, but the state is not yet at Running, while the Actor actually is
not idle.

Thanks Raim! Best rubber duck ever!

The test reliable segfaulted when a debug build as used, because it
constructed a counting semaphore within the C main function.

to assert that this function is only used when there is a current
fiber (e.g., not from the C main function).

It appears that gcc does not support __attribute__((unused)) on class
members and thus emits an warning when he finds it.

Fixes gcc non-stats builds.

Waaaaaaahhhhhhhhhhhhhhhhhh!!