Give error message instead of crashing if mbox offset is lost.
Give error message instead of crashing if mbox offset is lost.

damoxc/dovecot,Distrotech/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,Distrotech/dovecot,Distrotech/dovecot,Distrotech/dovecot,damoxc/dovecot,LTD-Beget/dovecot,Distrotech/dovecot,LTD-Beget/dovecot,damoxc/dovecot,damoxc/dovecot,damoxc/dovecot