The "uv_connect_t connect_req;" member of "conn" structure in defs.h should not be inside "union t" because used in same time as "t.addr4" in connect request. So t.addr4 contents overwrites the t.connect_req.

I've moved connect_req on level up, and now socks server works under Windows too.
...
uv_connect_t connect_req;
union {
uv_getaddrinfo_t addrinfo_req;
uv_req_t req;
struct sockaddr_in6 addr6;
struct sockaddr_in addr4;
struct sockaddr addr;
char buf[2048]; /* Scratch space. Used to read data into. */
} t;
...