unpackStructValue: fix a overflow in opt
unpackStructValue: fix a overflow in opt

jmhodges/dns,cloudflare/dns,michaelharo/dns,Psiphon-Inc/dns,bboreham/dns,miekg/dns,modulexcite/dns,skynet/dns,tempbottle/dns,skyportsystems/dns,hugozhu/dns,bragr/dns,curtiszimmerman/dns,tsenart/dns,janstk/dns,ErikDubbelboer/dns,ezoic/dns,asergeyev/dns,letsencrypt/dns,josephyzhou/dns