Fixes a potential null pointer dereference in `esp_netif` when PPP mode is enabled.
In the Ethernet event handler, `esp_netif_dhcps_start()` and `esp_netif_dhcps_stop()`
are now only called if `netif` is non-null (in provisioning mode when
the actual TCP/IP stack from IDF is used, in work mode the `netif` is
null, since the trafic is simply forwarded between wireless and wired
networks without TCP/IP stack involved)
Closes https://github.com/espressif/esp-idf/issues/14816
David Cermak