Fix Pipeline NPE or sth with multi

* followings are now throwing JedisDataException: it was uncontrolled or controlled by Redis itself
** exec without multi
** discard without multi
** multi within multi
* updates unit test

actually Redis returns ERR and we can pick, but Pipeline + multi has some complex sequence
so it can just throw NPE without ERR
Merge pull request #626 from HeartSaVioR/fix-pipeline-throws-npe-without-multi

Fix Pipeline throws NPE using exec without multi (fixes #623)