Summary:
InsertPathnameToSizeBytes() is called on shared/ and shared_checksum/ directories, which only exist for certain configurations. If we try to list a non-existent directory's contents, some Envs will dump an error message. Let's avoid this by checking whether the directory exists before listing its contents.
Closes https://github.com/facebook/rocksdb/pull/1895
Differential Revision: D4596301
Pulled By: ajkr
fbshipit-source-id: c809679