Don't overwrite the alert value if there is no alert to send

The function tls_early_post_process_client_hello() was overwriting the
passed "al" parameter even if it was successful. The caller of that
function, tls_post_process_client_hello(), sets "al" to a sensible default
(HANDSHAKE_FAILURE), but this was being overwritten to be INTERNAL_ERROR.
The result is a "no shared cipher" error (and probably other similar errors)
were being reported back to the client with an incorrect INTERNAL_ERROR
alert.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3314)