Don't crash if something tries to write 0 bytes in headers while saving the message. This happened sometimes with mbox.
Don't crash if something tries to write 0 bytes in headers while saving the
message. This happened sometimes with mbox.

--HG--
branch : HEAD

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