Fix bad array index test in Differential package code
Summary: This needs an isset() for cases when authority and packages don't completely overlap.
Test Plan:
- With a package set to trigger autoreview, created a revision.
- Observed error log, saw no more error.
- Saw package trigger autoreview properly.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16398