Summary:
Calling PurgeObsoleteFiles with a JobContext for which HaveSomethingToDelete
is false is a precondition violation. This would trigger an assertion in debug builds;
however, in release builds with assertions disabled, this can result in the
pending_purge_obsolete_files_ counter in DBImpl underflowing, which in turn can lead
to the process hanging during database close.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5448

Differential Revision: D15792569

Pulled By: ltamasi

fbshipit-source-id: 82d92c9b4f6a9efcdc69dbb3d5a52a1ae2dd2472