fix differential style break if you cancel inline comment before posting

Summary: see title. Thanks for the report @poop

Test Plan: loaded up FF, left a comment but did not submit and instead cancelled - observed sane UI

Reviewers: epriestley, vrana, poop

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2460

Differential Revision: https://secure.phabricator.com/D4728