slurmctld: test job_specs->min_nodes before altering the value via partition setting

This fixes a problem when trying to move a pending job from one partitition to another
while not supplying any other parameters:
 * if a partition value is present, the job is pending and no min_nodes are supplied,
   the job_specs->min_nodes get set from the detail_ptr value;
 * this causes subsequent tests for job_specs->min_nodes ==/!= NO_VAL to fail.

The following illustrates the behaviour, the example is taken from our system:
  palu2:0 ~>scontrol update jobid=3944 partition=night
  slurm_update error: Requested operation not supported on this system

  slurmctld.log
  [2011-04-06T14:39:51] update_job: setting partition to night for job_id 3944
  [2011-04-06T14:39:51] Change of size for job 3944 not supported
  [2011-04-06T14:39:51] updating accounting
  [2011-04-06T14:39:51] _slurm_rpc_update_job JobId=3944 uid=21215: Requested operation not supported on this system

==> The 'Change of size for job 3944' reveals that the !select_g_job_expand_allow() case was triggered,
    after setting the job_specs->min_nodes due to supplying a job_specs->partition.

Fix:
====
 Since the test for select_g_job_expand_allow() is not dependent on the job state, moved it up, before
 the test for the job_specs->partition. At the same time, also moved the equality for INFINITE/NO_VAL
 min_nodes values to the same place.
 Tests for job_specs->min_nodes below the job_specs->partition setting depend on the job state, 
 - the 'Reset min and max node counts as needed, insure consistency' requires pending state;
 - the other remaining test is only for IS_JOB_RUNNING/SUSPENDED.