Websocket模式varchar字段为null时报内存错误

[Leckun]

varchar字段为null时报如下错误：
System.AccessViolationException:“Attempted to read or write protected memory. This is often an indication that other memory is corrupt.”
应该是这段代码没有判断为空或者长度为0导致复制溢出（Websocket模式下，其他模式正常）
IoTSharp.Data.Taos\Protocols\TDWebSocket\TaosWebSocketContext.cs
GetValuePtr的这段的代码中IntPtr rowptr = IntPtr.Add(rowdata, offset);这句报错
private WSDataRow GetValuePtr(int ordinal)
{
var col = lstpColInfoData[ordinal];
int offset = 0;
short len = 0;
IntPtr data = IntPtr.Zero;
IntPtr rowdata = col.pData;
if (col.info.IS_VAR_DATA_TYPE() )
{
offset = col.varmeta.offset[_index];
IntPtr rowptr = IntPtr.Add(rowdata, offset);
len = Marshal.ReadInt16(rowptr, 0);
data = IntPtr.Add(rowptr, sizeof(Int16));
}

经分析，当varchar字段为空时offset=-1，rowdata=0x00000000，再复制就会导致指针错误，在if (col.info.IS_VAR_DATA_TYPE())前应该加个判断，如下

        if (rowdata == IntPtr.Zero)
            return new WSDataRow(TDengineDataType.TSDB_DATA_TYPE_NULL, rowdata, 0 , 0);

        if (col.info.IS_VAR_DATA_TYPE())
        {