Steps to reproduce:
1. fork any repo
2. start any pull request and get it merged
3. delete your own pull
4. from the user menu, click "My Pull Requests" (URL: https://hostname/user/[username]/requests)
5. click "All"

What happens:
Any pull requests from deleted repos are gone after the fork repo is being deleted.

What should happen (Expected behavior):
Keep all pull requests unless the base repo is being deleted.

Affected version:
The 3.10.1 installation on src.fedoraproject.org as of today.