Fix for Phragment ZIP controller

Summary: This applies a minor fix for the Phragment ZIP controller where it would raise missing index errors when attempting to map deleted fragments to file PHIDs (since deleted fragments no longer have files).

Test Plan: Tested this patch on a live server and saw the issue go away.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8243