fix a bug in the jstcss guard

test plan:
with guard running:
edit a handlebars file that has a corresponding
sass file.
e.g.: app/views/jst/FolderTreeItem.handlebars

it should not crash guard

Change-Id: Icd04510dc4ac0ff80674d1b3116da0dccb199fd9
Reviewed-on: https://gerrit.instructure.com/41019
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>