apps/speed: fix segfault while looking up algorithm name

The backport of master commit 5c6a69f5 (apps/speed: fix possible OOB
access in some EC arrays) as 1.1.0 commit 4e079413

 introduced a
regression.  The ecdh_choices array is iterated using an element count
but is NULL terminated.  This means that running 'openssl speed somealgo'
will result in a segfault when opt_found hits the NULL entry.

Fixes #8243

CLA: trivial

Signed-off-by: Jeff Mahoney <jeffm@suse.com>

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8244)