Suppress missing return value check warning in |EVP_DigestFinal|.

This is harmless, but it wasn't annoted with |(void)| so Coverity
complained about it.

Change-Id: Ie3405b0c0545944d49973d4bf29f8aeb6b965211
Reviewed-on: https://boringssl-review.googlesource.com/5612
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>