Summary:
Fix missing null check.
It is possible for a thread to successfully CAS count from >= threshold to 0 and also pop an empty list.
Example with 2 threads:
- Batch contains Threshold - 1 objects
- Thread 1 pushes an object, finds count == Threshold, CAS-es count to 0 successfully.
- Thread 2 pushes Threshold more objects, finds count == Threshold, CAS-es count to 0 successfully.
- Thread 1 pops all 2xThreshold objects (that's ok)
- Thread 2 pops 0 objects (that's ok too but we need to do null check before dereferencing the head of the list).

Reviewed By: davidtgoldblatt

Differential Revision: D17680391

fbshipit-source-id: a1d68997668077cc0b34cc66405628ed2f95702b