auth: Don't crash in non-plaintext auth if master user login is tried without master passdbs.
auth: Don't crash in non-plaintext auth if master user login is tried without master passdbs.

jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot