[3.9] krb5: Ignore password attributes for S4U2Self requests (CVE-2018-20217)

A Reachable Assertion issue was discovered in the KDC in MIT Kerberos 5 (aka krb5) before 1.17. If an attacker can obtain a krbtgt ticket using
an older encryption type (single-DES, triple-DES, or RC4), the attacker can crash the KDC by making an S4U2Self request.

References:

https://nvd.nist.gov/vuln/detail/CVE-2018-20217
http://krbdev.mit.edu/rt/Ticket/Display.html?id=8763

Patch:

https://github.com/krb5/krb5/commit/5e6d1796106df8ba6bc1973ee0917c170d929086

(from redmine: issue id 9802, created on 2018-12-27, closed on 2019-01-09)

main/krb5: upgrade to 1.15.4, security fix for CVE-2018-20217

Fixes #9802