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