sendmail: fix auth with cyrus-sasl-2.1.28

Apply patch extracted from sendmail-8-17.1.9 snapshot by dinoex@.

PR: 262935
Reviewed by: gshapiro
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37119

(cherry picked from commit c898b54affbaa9fc22c1d1b1de4a175dd250f37e)