Fix coverity-1610057

Fix coverity-1610057

Coverity caught a error in a recent change, in which atoi was used to
assign a value to two size_t variables, and then checked them for being
>= 0, which will always be true.

given that atoi returns an undefined value (usually zero) in the event
of a failure, theres no good way to check the return value of atoi for
validitiy.

Instead use OPENSSL_strtoul and confirm both that the translation
passed, and that the endptr value is at the NULL terminator (indicating
that the entire string was consumed)

Fixes openssl/private#552

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24861)