speed: range check the argument given to -multi

speed: range check the argument given to -multi

For machines where sizeof(size_t) == sizeof(int) there is a possible overflow
which could cause a crash.

For machines where sizeof(size_t) > sizeof(int), the existing checks adequately
detect the situation.

Fixes #16899

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16904)