i got a bug, at get value

[LiaoRiYang]

if i use CVAT create tfrecord, when i read it will work, but some tfrecord, will got wrong. https://github.com/vahidk/tfrecord/blob/5034c32f5acb91d72d5707e99521b6bf0aea1ca3/tfrecord/reader.py#L121

then i find sometime it (value) will be a empty list, so i changed

if len(value)==0:
    value = value
else:
    value = value[0]