visit_next_list() returns non-null on success, null on failure. The
comment's phrasing "until NULL return or error occurs" is needlessly
confusing. Scratch the "or error occurs" part.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190802122325.16520-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>