Negative numbers in "Children Job Summary" for periodic tasks

### Nomad version
Nomad v0.8.6 (fcc4149c55399eb4979cd3fe3fb983cfe6c8449a+CHANGES)

### Issue
```
vagrant@consulnomad-1:/tmp/nomadserver$ nomad status vault_debug-02
ID = vault_debug-02
Name = vault_debug-02
Submit Date = 2018-09-27T02:41:29+03:00
Type = batch
Priority = 50
Datacenters = test
Status = running
Periodic = true
Parameterized = false
Next Periodic Launch = 2018-09-27T16:50:00Z (4m49s from now)

Children Job Summary
Pending Running Dead
-1 -1 2

Previously Launched Jobs
ID Status
vault_debug-02/periodic-1538005800 dead
vault_debug-02/periodic-1538050665 dead
```

is it normal that `pending` and `running` are negative

Reproduce the report with Nomad v0.8.6 by running `nomad status` for a periodic batch job and inspect the Children Job Summary alongside its previously launched jobs. Determine whether negative Pending and Running counts are expected; done means confirming the behavior or identifying the status output that needs correction.

Written by the indexing model from the issue text.