Deallocate previously loaded SSL CONF module data

If application explicitly calls CONF_modules_load_file() the SSL
conf module will be initialized twice and the module data would leak.
We need to free it before initializing it again.

Fixes #6835

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6948)