lib-storage: mail_update_pvt_modseq() shouldn't crash if there is no private index.
lib-storage: mail_update_pvt_modseq() shouldn't crash if there is no private index.

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