Prevent select/cons_tres abort

Without this patch, test39.7 would cause _gen_combs() in
src/plugins/select/cons_tres/dist_tasks.c would abort due to a NULL
board_combs argument, which was due to ncomb_brd being zero. This
problem was due to some other inssue in cons_tres currently under
investigation, but this at least prevents the abort.

Relevent configuration information from slurm.conf:
SelectType=select/cons_tres
SelectTypeParameters=CR_Core_Memory,CR_CORE_DEFAULT_DIST_BLOCK,CR_ONE_TASK_PER_CORE
GresTypes=gpu
NodeName=psg-dgx2-01 NodeAddr=jette NodeHostName=jette RealMemory=1536000 Gres=gpu:16 Sockets=2 CoresPerSocket=24 ThreadsPerCore=2 State=UNKNOWN
PartitionName=debug Nodes=ALL Default=YES MaxTime=INFINITE State=UP

gres.conf (CPUs parameters are recognized as bad here):
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty0 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty1 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty2 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty3 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty4 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty5 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty6 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty7 CPUs=0-23,48-71
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty8 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty9 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty10 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty11 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty12 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty13 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty14 CPUs=24-47,72-95
NodeName=psg-dgx2-01 Name=gpu File=/dev/tty15 CPUs=24-47,72-95