Fix bug in `WikiLinkFilter`.

Closes #18819

- [ ]  #18819 Wiki link filter exception
    - [x]  Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/6d169d36cabda783116bcb8e2e6f73254566a670/builds) passes
    - [ ]  Wait for merge
    - [ ]  Pick into 8.9

See merge request !4775