macidn punycode buffer overread

libcurl's URL API function

[curl_url_get()](https://curl.se/libcurl/c/curl_url_get.html) offers punycode

conversions, to and from IDN. Asking to convert a name that is exactly 256

bytes, libcurl ends up reading outside of a stack based buffer when built to

use the *macidn* IDN backend. The conversion function then fills up the

provided buffer exactly - but does not null terminate the string.

This flaw can lead to stack contents accidently getting returned as part of

the converted string.