Fix DH private key check.

Fix DH private key check.

A recent addition removed setting the dh private key length when
a safe prime group is used. The private key validation check was relying on this
being set for safe primes. Setting the upper bound no longer checks the
length if the value is zero.

This caused a failure in the daily build of acvp_tests.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15760)