Fix overflow when the buffer is really small
Fix overflow when the buffer is really small

jdef/dns,AlexCheong/dns,bragr/dns,letsencrypt/dns,tonnerre/golang-dns,bboreham/dns,jmhodges/dns,ErikDubbelboer/dns,curtiszimmerman/dns,FiloSottile/dns,benbearchen/dns,Psiphon-Inc/dns,hugozhu/dns,miekg/dns,flebarbe/dns,michaelharo/dns,rolandshoemaker/dns,tsenart/dns,cloudflare/dns,ym/go-dns,Git-Host/dns,asergeyev/dns,janstk/dns,tempbottle/dns,modulexcite/dns,josephyzhou/dns,ezoic/dns,skyportsystems/dns,skynet/dns