darwinssl: handle long strings in TLS certs (follow-up)

- Fix handling certificate subjects that are already UTF-8 encoded.

Follow-up to b3b75d17 from two days ago. Since then a copy would be
skipped if the subject was already UTF-8, possibly resulting in a NULL
deref later on.

Ref: https://github.com/curl/curl/issues/1823
Ref: https://github.com/curl/curl/pull/1831

Closes https://github.com/curl/curl/pull/1836