P2P: Be more careful with wpa_config_update_psk() call

Commit bb4d4deb4bbb7b094faa16fa41f1f49b062cbf22 introduced a code path
that could potentially end up calling wpa_config_update_psk() with NULL
passphrase. While that may not happen in practice, it is better to make
sure it doesn't happen since that function will dereference the
passphrase pointer unconditionally.

Signed-hostap: Jouni Malinen <j@w1.fi>

wpa_supplicant/p2p_supplicant.c

@@ -900,7 +900,7 @@ static void wpas_start_wps_go(struct wpa_supplicant *wpa_s,
 	ssid->psk_set = params->psk_set;
 	if (ssid->psk_set)
 		os_memcpy(ssid->psk, params->psk, sizeof(ssid->psk));
-	else
+	else if (ssid->passphrase)
 		wpa_config_update_psk(ssid);
 	ssid->ap_max_inactivity = wpa_s->parent->conf->p2p_go_max_inactivity;