plm_slurm_module.c: don't leave the extra fd to /dev/null open

Prior to r29058, this same logic was in place (i.e., ensure that the
extra fd to /dev/null is closed).  It looks like it was accidentally
removed in the ORTE conversion to the state machine in r29058.

This ''might'' have something to do with many hangs that we're seeing
in Cisco MTT with jobs that exhibit failure (e.g., call MPI_ABORT)...?

cmr=v1.8.2:reviewer=rhc