Summary:
Found with readability-container-size-empty
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Pull Request resolved: https://github.com/facebook/folly/pull/1293

Reviewed By: markisaa

Differential Revision: D19241094

Pulled By: yfeldblum

fbshipit-source-id: 9362e8dc5dc1458fcd2b27c11ce81eafe674bcd2