QoS limits enforcement: correct a bug with 0-valued per user used limits

QoS limits enforcement on the controller side is based on a list of used_limits
per user. When a user is not yet added to the list, which is common when the
controller is restarted and the user has no running jobs, the current logic is
to not check some of the "per user limits" and let the submission succeed.
However, if one of these limits is a zero-valued limit, the check chould
failed as it means that no job should be submitted at all as it would
necessarily result in a crossing of the limit.

This patch ensures that even when a user is not yet present in the per user
used_limits list, the 0-valued limits are correctly treated.
Merge branch 'slurm-2.5'

Conflicts:
	src/slurmctld/acct_policy.c