fix(ss): address review findings for PR #96 — nil guards, resource leaks, dead code

- Bump go-shadowsocks2 to v0.1.3 (new API symbols)
- Return error instead of nil when Target() is empty in TCP handler
- Reset wbuf unconditionally on write error to prevent unbounded growth
- Add nil guard on targetAddr before WriteTo to prevent panic
- Remove duplicate NewClientConfig call and dead ClientConfig literal
- Remove unused noDelay metadata field
- Fix buffer-size guard to catch negative values (len(b)==0 → bufSize<=0)
- Add address context to parse/session error messages

handler/ss/handler.go

@@ -149,7 +149,7 @@ func (h *ssHandler) Handle(ctx context.Context, conn net.Conn, opts ...handler.H
 		target = tc.Target()
 	}
 	if len(target) == 0 {
-		return
+		return errors.New("ss: target not available")
 	}
 	conn = wrappedConn