BUG/MINOR: ssl: fix memory leak in ssl_fc_crtname by using SSL_CTX ex_data index

BUG/MINOR: ssl: fix memory leak in ssl_fc_crtname by using SSL_CTX ex_data index

The ssl_crtname_index was registered with SSL_get_ex_new_index() but the
certificate name is stored on a SSL_CTX object via SSL_CTX_set_ex_data().
The free callback is only invoked for the object type matching the index
registration, so the strdup'd name was never freed when the SSL_CTX was
released.

Fix this by using SSL_CTX_get_ex_new_index() instead, which ensures the
free callback fires when the SSL_CTX is destroyed.

No backport needed.