Fifo fixes

[HHHartmann]

Fixes problems I had during development.

Make sure all boxes are checked (add x inside the brackets) when you submit your contribution, remove this sentence before doing so.

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [ ] The code changes are reflected in the documentation at docs/*.

To be completed below: Description of and rationale behind this PR.

This contains fixes for problems I found while using fifo.

[marcelstoer]

@HHHartmann as it's been approved, do you want to merge this? Or do you still plan to add changes given @nwf comments?

[HHHartmann]

Nope, not planning to add changes.