This was broken in commit d177a8b5353f

ssh-agent: Fix leak in search_keys_like_attributes()
gck_enumerator_next() return value must be unref'fed as it's transfer full.

ssh foo.example.com is always asking to unlock the corresponding key while before this commit this works fine. 

gkd-ssh-agent-ops.c:op_sign_request has:
result = unlock_and_sign (session, key, mech, hash, n_hash, &n_result, &error);
g_object_unref (session);
g_object_unref (key);

After the leak fix mentioned above, unreffing 'key' causes the last reference to be released, so it's destroyed, and this locks the just unlocked key.
Removing the g_object_unref() avoids this issue, but readds a leak. I assume 'key' should be added somewhere so that it's kept alive, but I'm not really sure where/how :(