SegmentFault when repeatedlly call method `convert`

[xiaooloong]

save the following code to a.lua

local iconv = require 'resty.iconv'
local i = iconv:new('GBK', 'UTF-8')
while true do
    local t = i:convert('中文')
end

run it with the command luajit a.lua will simply result in a segmentation fault