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

ssl/ssl_lib.c

@@ -1217,6 +1217,8 @@ void SSL_free(SSL *s)
     OPENSSL_free(s->ext.ocsp.resp);
     OPENSSL_free(s->ext.alpn);
     OPENSSL_free(s->ext.tls13_cookie);
+    if (s->clienthello != NULL)
+        OPENSSL_free(s->clienthello->pre_proc_exts);
     OPENSSL_free(s->clienthello);
     OPENSSL_free(s->pha_context);
     EVP_MD_CTX_free(s->pha_dgst);