macidn punycode buffer overread

libcurl’s URL API function
curl_url_get() 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 accidentally getting returned as part of
the converted string.