istream-seekable: Fixed handling "buffer full" i_stream_read() result from underlying streams.
istream-seekable: Fixed handling "buffer full" i_stream_read() result from underlying streams.

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