FILS: Fix fils_cache_id check

This fixes the following compiler warning:
wpa_auth.c:4249:34: error: address of array 'a->conf.fils_cache_id'
 will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/ap/wpa_auth.c

@@ -4246,7 +4246,7 @@ static int wpa_auth_fils_iter(struct wpa_authenticator *a, void *ctx)
 {
 	struct wpa_auth_fils_iter_data *data = ctx;
 
-	if (a == data->auth || !a->conf.fils_cache_id ||
+	if (a == data->auth || !a->conf.fils_cache_id_set ||
 	    os_memcmp(a->conf.fils_cache_id, data->cache_id,
 		      FILS_CACHE_ID_LEN) != 0)
 		return 0;