pmrep et al: avoid issues with vanishing instances
pmrep et al: avoid issues with vanishing instances

There's a small window for a race condition where an instance is
being added to PMFG but it vanishes during the operation. Restore
handling of this corner case which was removed as part of commit
8a137fa, testing this automatically seems hard but manual test
case would be:

1) Add sleep(10) just prior the code block adding instances to PMFG
2) Start pmrep with instances that will be present and not present:

$ pmrep -i '.*top.*' -i '.*pmcd.*' -C proc.psinfo.cmd

3) During the artificial sleep stop an instance included with pmrep

Without this patch:

$ pmrep -i '.*top.*' -i '.*pmcd.*' -C proc.psinfo.cmd
pmrep: Unknown or illegal instance identifier

With this patch:

$ pmrep -i '.*top.*' -i '.*pmcd.*' -C proc.psinfo.cmd
  p.p.cmd
  000865

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp