Do not depend on the exact exit failure value of dgst app
Do not depend on the exact exit failure value of dgst app

On most platforms the EXIT_FAILURE is 1 but on NonStop platform
the EXIT_FAILURE is -1 truncated to 255.

Fixes #15633

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15694)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl