userdb-passwd: Fix getpwent errno handling

userdb-passwd: Fix getpwent errno handling

In https://bugs.gentoo.org/667118 Reuben Farrelly
noticed that running
    # doveadm user '*'
causes auth daemon to generate errors like:
    auth-worker(3585): Error: getpwent() failed: Invalid argument

This happens because on successful call getpwent()
now sets errno=EINVAL starting from glibc-2.28.
See https://sourceware.org/PR16004 for details.

The fix is to reset 'errno' before 'getpwent()' is called.

Reported-by: Reuben Farrelly
Bug: https://bugs.gentoo.org/667118
Bug: https://sourceware.org/PR16004
Original-Author: Sergei Trofimovich <slyfox@gentoo.org>