PHP 8.0: fix array-list detection

The `nNextFreeElement` field is initialized with `ZEND_LONG_MIN` instead
of zero since PHP 8.0. Adjust our `php_mp_is_hash()` check accordingly.
See [1] and [2] for details.

NB: PHP 8.1 introduces `zend_array_is_list()`, which may be used here.
See [3] and [4] for details.

[1]: https://wiki.php.net/rfc/negative_array_index
[2]: php/php-src#3772
[3]: https://wiki.php.net/rfc/is_list
[4]: php/php-src#6070

Since I don't observe any other problems on PHP 8.1, closing the
relevant issue.

Fixes #171

Author: Mikhail Galanin

src/tarantool_msgpack.c

@@ -96,7 +96,10 @@ void php_mp_pack_array(smart_string *str, size_t len) {
 int php_mp_is_hash(zval *val) {
 	HashTable *ht = Z_ARRVAL_P(val);
 	int count = zend_hash_num_elements(ht);
-	if (count != ht->nNextFreeElement) {
+	if (count == 0) {
+		/* An empty array is considered as a list. */
+		return 0;
+	} else if (count != ht->nNextFreeElement) {
 		return 1;
 	} else {
 		HashPosition pos = {0};