speed.c: Check for 0 block size

speed.c: Check for 0 block size

Although this cannot really happen check for 0 block size
to avoid division by 0.

Fixes Coverity 1633936

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25822)