MFC r320906: MFV r320905: Import upstream fix for CVE-2017-11103.

MFC r320906: MFV r320905: Import upstream fix for CVE-2017-11103.

In _krb5_extract_ticket() the KDC-REP service name must be obtained from
encrypted version stored in 'enc_part' instead of the unencrypted version
stored in 'ticket'.  Use of the unecrypted version provides an
opportunity for successful server impersonation and other attacks.

Submitted by:	hrs
Obtained from:	Heimdal
Security:	FreeBSD-SA-17:05.heimdal
Security:	CVE-2017-11103

git-svn-id: svn://svn.freebsd.org/base/stable/10@320907 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f