Prevent etcd from crashing when given a bad grant payload, e.g.:
$ curl -d '{"name": "foo"}' http://localhost:2379/v3/auth/role/add
{"header":{"cluster_id":"14841639068965178418", ...
$ curl -d '{"name": "foo"}' http://localhost:2379/v3/auth/role/grant
curl: (52) Empty reply from server
Signed-off-by: Gyuho Lee <leegyuho@amazon.com>