fix seg when mod unload with event_sink listeners present

src/mod/event_handlers/mod_event_socket/mod_event_socket.c

@@ -598,6 +598,8 @@ static void send_disconnect(listener_t *listener, const char *message)
 		switch_snprintf(disco_buf, sizeof(disco_buf), "Content-Type: text/disconnect-notice\nContent-Length: %d\n\n", mlen);
 	}
 
+	if (!listener->sock) return;
+
 	len = strlen(disco_buf);
 	switch_socket_send(listener->sock, disco_buf, &len);
 	if (len > 0) {