apps/speed: fix possible OOB access in some EC arrays

because there are actually 18 curves defined, but only 16 are plugged for
ecdsa test.
Deduce array size using OSSL_NELEM and so remove various magic numbers,
which required some declarations moving.
Implement OPT_PAIR list search without a null-ending element.
Fix some comparison between signed and unsigned integer expressions.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6133)