When determine if a job can fit into a TRES time limit after resources

have been selected set the time limit appropriately if the job didn't
request one.

If the partition has no DefaultTime setting, and no time_limit was given
for the job, job_ptr->time_limit == NO_VAL.

With AccountingStorageEnforce=safe this will prevent jobs from ever
starting if the association has any limit set for CPUMins.
(NO_VAL * cpus is a very large number, but if no time_limit is given
anywhere that is what they get :))

Bug 2388.