[Shenandoah] Incomplete SATB buffers may not be processed during degenerated young collection

When Shenandoah's young generation exhausts available memory, it transitions to a "degenerated" cycle. If old marking is in progress, SATB queues need to be drained before evacuation to avoid invalidating any pointers in the queues. In the case when old marking is active and the concurrent collection failed during root scan, the SATB queues might not be drained - this is an error.