Fix segv in slurmctld for job step with relative option

Here is the way to reproduce it :
[root@cuzco27 georgioy]# salloc -n64 -N4 --exclusive
salloc: Granted job allocation 8
[root@cuzco27 georgioy]#srun -r 0 -n 30 -N 2 sleep 300&
[root@cuzco27 georgioy]#srun -r 1 -n 40 -N 3 sleep 300&
[root@cuzco27 georgioy]# srun: error: slurm_receive_msg: Zero Bytes were transmitted or received
srun: error: Unable to create job step: Zero Bytes were transmitted or received