Free pre_proc_exts in SSL_free()

Free pre_proc_exts in SSL_free()

Usually it will be freed in tls_early_post_process_client_hello().
However if a ClientHello callback will be used and will return
SSL_CLIENT_HELLO_RETRY then tls_early_post_process_client_hello()
may never come to the point where pre_proc_exts is freed.

Fixes #12194

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/12330)

(cherry picked from commit 94941cada25433a7dca35b5b9f8cbb751ab65ab3)