go.net/proxy: fix desired destination address in SOCKS5 CONNECT

Both types IPv6 IPv4-mapped address and IPv4-compatible
address are not allowed to be used in wire protocols.

Fixes golang/go#4709.

Thank you raptium for original CL 6922050.

R=golang-dev, agl
CC=golang-dev, raptium
https://golang.org/cl/7220047
2 files changed