R1:15021a0bcc91

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