Prevent memory corruption at slurmstepd shutdown

This has been seen only in slurmstepd with --enable-memory-leak-debug
enabled at slurmstepd shutdown time on the smd cluster with
Linux 3.8.0-23-generic #34-Ubuntu SMP Wed May 29 20:22:58 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux