If mbox file contains only single LF, treat it as an invalid mbox file instead of breaking later.
If mbox file contains only single LF, treat it as an invalid mbox file
instead of breaking later.

--HG--
branch : HEAD

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