Fix S390X bad size_t that causes memory trash in legacy ciphers

Fix S390X bad size_t that causes memory trash in legacy ciphers

This caused a SEGV inside tls13_enc() when using chacha_poly.
The tls code assigns the iv_length to a size_t (even though it is an int).
This is actually really bad since it could be -1, which will then trash the iv buffer.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9890)