mbox: Don't crash when expunging all messages and file doesn't end with [CR]LF.
mbox: Don't crash when expunging all messages and file doesn't end with [CR]LF.

--HG--
branch : HEAD

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