[sgen-gray-queue] Always update queue->cursor when dequeing

Previously, queue->cursor would not be set to NULL if the last element would be dequeued and queue->first would be set to NULL.
[sgen-gray-queue] Fix crashing sgen-domain-unload sgen test

sgen-domain-unload.exe test would crash because of a bug involving queue->alloc_prepare_func => workers_gray_queue_share_redirect => sgen_gray_object_dequeue_section on queue => set queue->first->size depending on queue->cursor. The solution is simply to make sure that the cursor is set on the queue, before allocating a new section, even if it is does not seems necessary in the first place.