virtual: Don't crash if trying to change a message that was already expunged in backend mailbox.
virtual: Don't crash if trying to change a message that was already expunged in backend mailbox.

--HG--
branch : HEAD

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