main/cyrus-sasl: (db_gdbm) fix gdbm_errno overlay from gdbm_close

gdbm_close also sets gdbm_errno since version 1.17. This leads to a problem in libsasl as the gdbm_close incovation overlays the gdbm_errno value which is then later used for the error handling.

This leads to inability to authenticate:

Feb 21 02:34:17 3a146eb8f574 postfix/smtpd[87]: warning: SASL authentication failure: Couldn't fetch entry from /etc/sasl2/sasldb2
Feb 21 02:34:17 3a146eb8f574 postfix/smtpd[87]: warning: SASL authentication failure: Password verification failed
Feb 21 02:34:17 3a146eb8f574 postfix/smtpd[87]: warning: unknown[172.17.0.1]: SASL PLAIN authentication failed: generic failure

Upstream: https://github.com/cyrusimap/cyrus-sasl/pull/554

See also:

• https://bugs.archlinux.org/task/59873
• https://bugs.gentoo.org/677200