mdbox: Don't assert-crash when having to open mail file during specific copying situations.
mdbox: Don't assert-crash when having to open mail file during specific copying situations.

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