1.x: fix Spsc queues reporting not empty but then poll() returns null

[akarnokd]

In the spsc queues, the indexes were written before the actual elements and thus a concurrent isEmpty check would report a non-empty queue but a poll would still return null. The fix swaps the two writes.

Note that this an inconsistency in the original JCTools code and not the lack of keeping up with it.

[artem-zinnatullin]

👍

[stevegury]

👍