Fix issue where heterogeneous jobs were not properly purged in some cases.

Issue could be reproduced by restarting slurmctld after a heterogeneous job
finished, but before MinJobAge time passed. Since the pack_job_list
job_record memeber wasn't saved/loaded to/from the job_state, the function
_validate_pack_jobs() is responsible for rebuilding the pack_job_list. Issue
was that the function was skiping the rebuild work for finished jobs, thus
other functions like the thread responsible for purging old jobs was failing
to iterate over a NULL pack_job_list which was never rebuilt.

Bug 4383.