Prevent double-free of CTLOG public key

Previously, if ct_v1_log_id_from_pkey failed, public_key would be freed by
CTLOG_free at the end of the function, and then again by the caller (who
would assume ownership was not transferred when CTLOG_new returned NULL).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>