[util] clear OpenSSL error queue on jwt-cpp activity

As it turns out, jwt-cpp doesn't clean up OpenSSL error queue in case
of errors.  While I think it's a good idea to correct that improper
OpenSSL API usage in the jwt-cpp library itself (and I'm going to work
on a patch for jwt-cpp library upstream), I updated Kudu's jwt-util
code to report on those errors and clean the error queue as an interim
solution.

I haven't added any tests, but a few tests in a follow-up patch would
fail without these changes.

This is a follow-up to 5a3d116f302bde07e86bf80c237f8a595d5003b4.

Change-Id: I1a18a0d78bca0ea32878dc3825523e1bd6ecb019
Reviewed-on: http://gerrit.cloudera.org:8080/19895
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Wenzhe Zhou <wzhou@cloudera.com>
Reviewed-by: Zoltan Chovan <zchovan@cloudera.com>
Reviewed-by: Attila Bukor <abukor@apache.org>
3 files changed