signed_descriptor_digest has a length of DIGEST_LEN but the memset
used to fill it used DIGEST256_LEN.
Signed-off-by: Kris Katterjohn <katterjohn@gmail.com>
Additionally, use it to test that is_staledesc is set correctly.
Eventually we'll want to test all the other flags, but I'm aiming
for only adding coverage on the changed code here.