GetNext() should always set eos.

This was breaking the ASAN build because the select node wasn't
initializing eos. I added this to a couple of other exec nodes too.

Change-Id: If46907cd72f76cf3c7324caa628cc05d76be3efa
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4929
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins
(cherry picked from commit c9b65ee52689ab33a06124388b1cf679f9756982)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4946
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>