negative pull request count

[cevin]

• Gitea version (or commit ref): 1.9.0-rc2
• Git version: 2.9.5
• Operating system: centos
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant

[lunny]

duplicated with #7428 and fixed by #7518