io_add_istream(): Allow the istream to be added even if it doesn't have fd.
io_add_istream(): Allow the istream to be added even if it doesn't have fd.

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