lib-mail: Don't assert-crash when multipart doesn't actually have any parts.
lib-mail: Don't assert-crash when multipart doesn't actually have any parts.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot