fix a bug about creating task file

Originally, it will report error "parallel: Error: Cannot open gnuparallel_hostfile.3040659.task.". 
It makes sense because the $n can equals to 0 if NB_CORE is less than NB_CORE_PER_TASK.
Consequently [ $n -gt 0 ] will yield false, which will skip the task file creation process.
Since the task file is not created,  it will report error when using it.