crashes occurs in long string

[robertlzj]

for count=1,8000 do
	print(count)
	stringGBK=gbk.fromutf8(string.rep('a',count))
	--last count is 4097
end
error'should but wont execute here, and crash without error message'

last count is 4097 when repeat 'a', 2049 - 'aa', 1366 - 'a啊'/'aaa'