mbox: Fixed crash with pop3_lock_session=yes if dotlock already existed.
mbox: Fixed crash with pop3_lock_session=yes if dotlock already existed.

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