salloc/sbatch: make initialization of ntasks_per_node consistent

sbatch initializes ntasks_per_node with 0, while salloc uses NO_VAL.

Both eventually make an allocation based on these parameters. For
consistency, this patch ensures that ntasks_per_node is initialized
with 0 in both programs (there is no 'ntasks_per_node_set', hence
0 was likely meant to indicate 'not set by user').