Don't crash if no headers are given (fixes FETCH BODY[n.MIME] crashing)
Don't crash if no headers are given (fixes FETCH BODY[n.MIME] crashing)

--HG--
branch : HEAD

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