Fix for slurmstepd setfault

With slurm-15.08.10, we're seeing occasional segfaults in slurmstepd. The logs point to the following line: slurm-15.08.10/src/slurmd/slurmstepd/mgr.c:2612

On that line, _get_primary_group() is accessing the results of getpwnam_r():
    *gid = pwd0->pw_gid;

If getpwnam_r() cannot find a matching password record, it will set the result (pwd0) to NULL, but still return 0. When the pointer is accessed, it will cause a segfault.

Checking the result variable (pwd0) to determine success should fix the issue.