"Already forked" logic doesn't account for guest user

Sentry: https://sentry.gitlap.com/gitlab/gitlabcom/issues/1178/ - undefined method 'already_forked?' for nil:NilClass

Line: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/projects/forks/index.html.haml#L32

mentioned in issue #13350 (closed)

mentioned in merge request !2793 (merged)

Status changed to closed by commit dd4171eb

mentioned in commit dd4171eb