Prevent slurmstepd ABRT when parsing gres.conf CPUs.

Calling bit_unfmt() with a zero bit_size() bitmap leads to a later
call to bit_nclear() with start=0 and stop=-1, leading to the ABRT.

This scenario happened when cgroup.conf has ConstrainDevices=yes and
task_cgroup_devices_create() tries to collect the GRES devices
but gres_cpu_cnt=0, thus creating a p->cpus_bitmap = bit_alloc(gres_cpu_cnt);
of zero size which is passed by argument to bit_unfmt().

gres_cpu_cnt is 0 because we have defined a gres.conf like this:

Name=gpu Type=tesla File=/tmp/gres/tesla0 CPUs=0,1
Name=gpu Type=tesla File=/tmp/gres/tesla1 CPUs=0,1
Name=gpu Type=kepler File=/tmp/gres/kepler0 CPUs=2,3
Name=gpu Type=kepler File=/tmp/gres/kepler1 CPUs=2,3

but have no GresTypes nor GRES option in the slurm.conf / node config def.

Bug 3974