Sync Pull Requests fails if there is a pull request where the forked repository is deleted.

[kumada626]

What happened:
In the Options tab, users click to Pull Request Sync button.
However, Screwdriver returns 500 error if there is a pull request where the forked repository is deleted.

What you expected to happen:
Correctly syncs pull requests without error.

How to reproduce it:

1. Make Pull Request from forked repository.
2. Delete forked repository. Pull Request still remains open.
3. Click Sync Pull Request button.