Fixed crash in mumu and bluestack emulators

[litefeel]

using UnityEngine;
using ZString;

public class zstringTest : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        using (zstring.Block())
        {
            var missionID = (zstring)"xxxxxx";
            var intv = (zstring)12345;
            // crash in mumu and bluestack emulators
            var zzzz = missionID + intv;
            Debug.LogError(zzzz.ToString());
        }
    }

}