Fix index out of range when decoding grayscale image.
Fix index out of range when decoding grayscale image.

The extra padding is required to decode a grayscale image which is same
way as for YCbCr image.

shirase-aoi/go-libjpeg,shirase-aoi/go-libjpeg,pixiv/go-libjpeg,pixiv/go-libjpeg,pixiv/go-libjpeg