Panic when viewing pull requests on a repo with a deleted fork

[ollien]

• Gitea version (or commit ref): b83114f
• Git version: 2.21.0
• Operating system: Linux (openSUSE Tumbleweed)
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL) https://try.gitea.io/ollien/bad-pr-target/pulls
  • No
  • Not relevant
• Log gist: https://gist.github.com/ollien/6f09291db568d30ad2ae69969357dba0

Description

1. Create a repository.
2. Fork that repository as another user.
3. Make a PR to the original repository.
4. Delete the fork.
5. The /pulls page of the original repo will now 500