restore max_nodes of desc to NO_VAL when checkpointing job

We're having some trouble getting our slurm jobs to successfully
restart after a checkpoint.  For this test, I'm using sbatch and a
simple, single-threaded executable.  Slurm is 2.5.4, blcr is 0.8.5.
I'm submitting the job using sbatch:

$ sbatch -n 1 -t 12:00:00 bin/bowtie-ex.sh

I am able to create the checkpoint and vacate the node:

$ scontrol checkpoint create 137
.... time passes ....
$ scontrol vacate 137

At that point, I see the checkpoint file from blcr in the current
directory and the checkpoint file from Slurm
in /var/spool/slurm-llnl/checkpoint.  However, when I attempt to
restart the job:

$ scontrol checkpoint restart 137
scontrol_checkpoint error: Node count specification invalid

In slurmctld's log (at level 7) I see:

[2013-05-29T12:41:08-07:00] debug2: Processing RPC: REQUEST_CHECKPOINT(restart) from uid=*****
[2013-05-29T12:41:08-07:00] debug3: Version string in job_ckpt header is JOB_CKPT_002
[2013-05-29T12:41:08-07:00] _job_create: max_nodes == 0
[2013-05-29T12:41:08-07:00] _slurm_rpc_checkpoint restart 137: Node count specification invalid
Merge branch 'slurm-2.5'