Fix two bugs:

- The node count in the bluegene select plugin job record was getting set to 
    zero, which prevented the job from re-running after a requeue
- Fix bug in new kill job logic which tried to set the job's start time in 
   the RPC from slurmctld to slurmd. We need to leave the value as zero rather 
   than using a NULL job pointer if the job is not found