Fix for job pn_min_cpus where logic adjusted job desc but not job record.

job_limits_check() uses the job desc to call _valid_pn_min_mem(). This
second function might adjust the following values (up to date):

cpus_per_task
pn_min_memory
min_cpus
max_cpus
pn_min_cpus

If the function returns success, these adjusted members need to be copied
back to the job_record. It turns out pn_min_cpus wasn't copied back,
thus the logs claimed to automatically increase pn_min_cpus but actually
the job record wasn't modified and the select plugin tried to allocate
wrong amount of resources.

Bug 4823.