Add DCHECK to PrintExecSummary() to prevent indexing into empty array

Change-Id: Ic4d21310a5cfbb8284f58bc0244572906ae050e4
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3017
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins