improve handling of issue comment hook

When an issue comment hook is invoked but the pull request is not yet known it should at least try to fetch it. The same checking is already done in some other places:

* https://github.com/dirk-thomas/ghprb/blob/b10530426f3061da2f9f4927369aaeda12344130/src/main/java/org/jenkinsci/plugins/ghprb/GhprbRepository.java#L111
* https://github.com/dirk-thomas/ghprb/blob/b10530426f3061da2f9f4927369aaeda12344130/src/main/java/org/jenkinsci/plugins/ghprb/GhprbRepository.java#L245