in ban_lurker_test_ban() we remove completed bans from the obans list
so the ban list tail might not be present in the obans list.

To ensure that we stop at the tail, we now find our obans iteration
termination ban walking the bans list backwards until we find an
active ban, the head or the first obans element. In the latter two
cases we are done.

If there are any bans left to mark completed, the last active ban must
be contained in the obans list so we should terminate correctly.

Fixes #2556