P2P: Add NULL pointer validation for ssid for group started event

It looks likely that ssid is not NULL here, but better keep this
consistent with other uses of ssid pointer in this function.

wpa_supplicant/p2p_supplicant.c

@@ -475,7 +475,7 @@ static void wpas_group_formation_completed(struct wpa_supplicant *wpa_s,
 		wpa_msg(wpa_s->parent, MSG_INFO, P2P_EVENT_GROUP_STARTED
 			"%s GO ssid=\"%s\" freq=%d passphrase=\"%s\" "
 			"go_dev_addr=" MACSTR "%s",
-			wpa_s->ifname, ssid_txt, ssid->frequency,
+			wpa_s->ifname, ssid_txt, ssid ? ssid->frequency : 0,
 			ssid && ssid->passphrase ? ssid->passphrase : "",
 			MAC2STR(go_dev_addr),
 			persistent ? " [PERSISTENT]" : "");