Fixed passdb credential lookups to work again with blocking passdbs.
Fixed passdb credential lookups to work again with blocking passdbs.

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branch : HEAD

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