Fix Coverity 1503218: negative loop bound

Fix Coverity 1503218: negative loop bound

OPENSSL_sk_num returns an integer which can theoretically be negative.
Assigning this to a size_t and using it as a loop bound isn't ideal.

Rather than adding checked for NULL or negative returns, changing the loop
index and end to int is simpler.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17954)