fix buffer.Truncate to handle non-zero resets, and remove detection of invalid usage
fix buffer.Truncate to handle non-zero resets, and remove detection of invalid usage

xladykiller/ffjson,trigrass2/ffjson,xyliuke/ffjson,xladykiller/ffjson,sunminghong/ffjson,cw2018/ffjson,trigrass2/ffjson,pquerna/ffjson,cmeon/ffjson,jamesliu96/ffjson,xyliuke/ffjson,cmeon/ffjson,cw2018/ffjson,xiuzhifu/ffjson,pquerna/ffjson,xiuzhifu/ffjson,sunminghong/ffjson,jamesliu96/ffjson