Fix allocate test in 2.8.fb.trunk.

Merging:
https://github.com/facebook/rocksdb/commit/c7076a7a0510eb69ced78ac7cdb03442c810e63c
and
https://github.com/facebook/rocksdb/commit/05c168658e60d9748fe873f316a2c0e076d6dd7b

into 2.8.fb.trunk