dgram: change 'this' to 'self' for 'isConnected'

The function 'isConnected' is directly called by many 'Socket' instance,
so we shouldn't directly use 'this' because 'this' will be the self of
function itself, and we should use 'self' as the instance of 'Socket'
function.

PR-URL: https://github.com/nodejs/node/pull/27338


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>