idn: fix WinIDN null ptr deref on bad host
- Return CURLE_URL_MALFORMAT if IDN hostname cannot be converted from
UTF-8 to UTF-16.
Prior to this change a failed conversion erroneously returned CURLE_OK
which meant 'decoded' pointer (what would normally point to the
punycode) would not be written to, remain NULL and be dereferenced
causing an access violation.
Closes https://github.com/curl/curl/pull/11983