Fix leaking FragmentExecStates.

The previous attempt to fix this resulted in the FragmentExecState never being
deleted.

Change-Id: I81afe9c60e557119d0451335219379fedb2021c8
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5273
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins