X509_CRL_digest() - ensure precomputed sha1 hash before returning it

X509_CRL_digest() didn't check if the precomputed sha1 hash was actually
present.  This also makes sure there's an appropriate flag to check.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2314)
(cherry picked from commit 6195848b)