Don't assert-crash if looking up a non-parsed header after headers were already parsed once.
Don't assert-crash if looking up a non-parsed header after headers were
already parsed once.

--HG--
branch : HEAD

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