Bug not in any released Tor.

Reported on tor-dev by Gisle Vanem.  Bug not in any released Tor

(The suggested patch used _MSC_VER, but that's not how we do stuff
with autoconf.  With autoconf, you detect the feature you want,
rather than trying to list all the systems that do or do not have
it.)

Closes ticket 28851.

In theory it would be better to detect this bug in advance, but this
approach is much simpler, and therefore safer to backport.

This closes tor issue 28973.

Otherwise, coverity complains at is.

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>