auth_request_set_fields(): Don't crash with empty fields.
auth_request_set_fields(): Don't crash with empty fields.

--HG--
branch : HEAD

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