Fix MR creation on fork of deleted project

The selected_target_project helper method is used for actions where target_project_id is a top-level parameter, not nested under the merge_request hash.

Even though that logic was wrong, if the target project wasn't a fork, or was a fork of an active project, this would work through coincidence (although it might check the wrong value for approvals_before_merge).

If a project was a fork of a deleted project, however, then selected_target_project would be nil, causing a 500 error. Ensure that there is always a target project to compare to.

Closes #732 (closed).