Fix multiple bugs with ctld's UCL parsing

• Don't segfault when parsing a misformatted auth-group section
• If the config file specifies a chap section within a target but no auth-group, create a new anonymous auth-group. That matches the behavior with non-UCL config files.
• Protect some potential segfaults with assertions

PR: 274380
MFC after: 1 week
Sponsored by: Axcient
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D43198