SAML login can't be aborted if final registration step is not completed

[fuzzylogic2000]

URL: diid-dev
user: saml test user
expected behaviour: If final registration step was skipped (e.g. by closing the browser window, or going back to the main site), the user is shown as logged in and clicking on logout will actually log them out
behaviour: Pressing logout won't log the user out and they have to complete the registration. Logging in with another account is impossible without clearing the browser cache if registration is not completed.
important screensize:
device & browser:
Comment/Question:

liqd/adhocracy-plus#1610

most likely connected to cd1d9ff