go.net/proxy: don't pass invalid domain name length to SOCKS5 proxies

SOCKS5 uses a single-byte field for domain name length. This change
causes dials to domain names longer than 255 chars to fail instead
of sending an invalid request to the proxy.

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/90790044
1 file changed