upstream: reset comment=NULL for each key in do_fingerprint();

fixes "no comment" not showing on when running `ssh-keygen -l` on multiple
keys where one has a comment and other following keys do not. Patch from
Markus Kuhn via GHPR407, bz3580

OpenBSD-Commit-ID: 3cce84456fdcd67dc6b84e369f92c6686d111d9b
