Fix select/cons_res task distribution bug

Fix bug in select/cons_res task distribution logic when tasks-per-node=0.
Eliminates misleading slurmctld message
"error:  cons_res: _compute_c_b_task_dist oversubscribe."
This problem was introduced in SLURM version 2.2.5 in order to fix
a task distribution problem when cpus_per_task=0. Patch from Rod Schultz, Bull.