Prevent unintialized memory being passed to write().

Syscall param write(buf) points to uninitialised byte(s)             (syscall-template.S:81)
  at 0x5289D5D: (syscall-template.S:81)
  by 0x58C2DB: stepd_connect (stepd_api.c:271)
  by 0x4294C9: _fill_registration_msg (slurmd.c:729)
  by 0x428E04: send_registration_msg (slurmd.c:615)
  by 0x42E4AE: slurmd_req (req.c:394)
  by 0x428CF1: _service_connection (slurmd.c:594)
  by 0x52830A4: start_thread (pthread_create.c:309)
  by 0x5592CFC: clone (clone.S:111)
Address 0x5f334c7 is 167 bytes inside a block of size 16,400 alloc'd
  at 0x4C2ABA0: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  by 0x464835: slurm_xmalloc (xmalloc.c:86)
  by 0x47E38A: init_buf (pack.c:156)
  by 0x58C04A: stepd_connect (stepd_api.c:243)
  by 0x4294C9: _fill_registration_msg (slurmd.c:729)
  by 0x428E04: send_registration_msg (slurmd.c:615)
  by 0x42E4AE: slurmd_req (req.c:394)
  by 0x428CF1: _service_connection (slurmd.c:594)
  by 0x52830A4: start_thread (pthread_create.c:309)
  by 0x5592CFC: clone (clone.S:111)
Address 0x5f334c7 is 167 bytes inside a block of size 16,400 alloc'dUninitialised value was created by
  at 0x4C2ABA0: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  by 0x464835: slurm_xmalloc (xmalloc.c:86)
  by 0x47E38A: init_buf (pack.c:156)
  by 0x58C04A: stepd_connect (stepd_api.c:243)
  by 0x4294C9: _fill_registration_msg (slurmd.c:729)
  by 0x428E04: send_registration_msg (slurmd.c:615)
  by 0x42E4AE: slurmd_req (req.c:394)
  by 0x428CF1: _service_connection (slurmd.c:594)
  by 0x52830A4: start_thread (pthread_create.c:309)
  by 0x5592CFC: clone (clone.S:111)