Don't explode when a ChangeDescription references an invalid diff.

Review Request #6094 — Created July 10, 2014 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
d59f28e...

Reviewers

When a diff is deleted from the database, it doesn't remove the field from any
ChangeDescription objects (since that's buried in a JSONField and there's no
relation between them). This meant that if a published revision of a diff was
deleted, rendering the ChangeDescription would explode.

This change adds some bullet-proofing around the DiffField rendering to avoid
doing anything if the associated diff doesn't exist.

Ran unit tests.