lib-storage: Handle better when attempting to access shared mailboxes for nonexistent users.
lib-storage: Handle better when attempting to access shared mailboxes for nonexistent users.

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