Fix: panic when getting RemoteAddr when upstream proxy in use

psiphon/TCPConn.go

@@ -61,12 +61,16 @@ func makeTCPDialer(config *DialConfig) func(network, addr string) (net.Conn, err
 		if err != nil {
 			return nil, ContextError(err)
 		}
-		if config.ResolvedIPCallback != nil {
-			host, _, err := net.SplitHostPort(conn.RemoteAddr().String())
-			if err == nil {
-				config.ResolvedIPCallback(host)
-			}
-		}
+		// Note: when an upstream proxy is used, we don't know what IP address
+		// was resolved, by the proxy, for that destination.
+		if config.ResolvedIPCallback != nil && config.UpstreamProxyUrl == "" {
+			remoteAddr := conn.RemoteAddr()
+			if remoteAddr != nil {
+				host, _, err := net.SplitHostPort(conn.RemoteAddr().String())
+				if err == nil {
+					config.ResolvedIPCallback(host)
+				}
+			}		}
 		return conn, nil
 	}
 }