Summary:
When using custom Env, trying to call DestroyDB() with default Options will
fail.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6539
Test Plan: ./db_stress
Differential Revision: D20476204
Pulled By: riversand963
fbshipit-source-id: 612c6754660cc9b5bb3e9c2dbb2f6ecd7f648797