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