Fix regression in 16.05.6 where if you request multiple cpus per task (-c2)

and request --ntasks-per-core=1 and only 1 task on the node
the slurmd would abort on an infinite loop fatal.

Regression is from commit 5265420d.

Without this fix you can get into an infinite loop in the task/affinity
plugin.  The loop is handled by producing a fatal.

Bug 3118