Fix checkpointing requeued jobs in a bad state

Requeued jobs are marked as PENDING|COMPLETING until the epilog checks
in. The issue is that if job_set_alloc_tres gets called while in the
PENDING|COMPLETING state, the job's alloc_tres_str will be free'd. If
this job then gets checkpointed in this state (PENDING|COMPLETING + no
tres_alloc_str) on startup the controller would crash because it
expected the job to have a tres_alloc_str/cnt when in the COMPLETING
state. This could be triggered if starting the controller without the
dbd up. When the dbd comes up, the assoc_cache_mgr calls
_update_job_tres() which calls job_set_alloc_tres. It could also be
triggered by adding new tres.

This most likely started happening in 17.11.5 because of commit
865b672f which introduced calling _update_job_tres() on each job
after the dbd comes up.

Bugs 5137,4522