Reproduce a panic related to batch GC
Reproduce a panic related to batch GC

Test case that reproduces a panic with the following stacktrace:

```
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x1149715]

goroutine 35 [running]:
testing.tRunner.func1(0xc0001e2200)
	/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:792 +0x387
panic(0x167e400, 0x1c43a30)
	/usr/local/Cellar/go/1.11.2/libexec/src/runtime/panic.go:513 +0x1b9
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-immutable-radix.(*Iterator).Next(0xc0003a4080, 0x17f7ba0, 0x0, 0xc0002e74a0, 0xc0003a0510, 0xc0003a0530, 0xc0003a0530)
	/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-immutable-radix/iter.go:81 +0xa5
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-memdb.(*radixIterator).Next(0xc0003a0420, 0x1756059, 0xb)
	/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-memdb/txn.go:634 +0x2e
github.com/hashicorp/nomad/nomad/state.(*StateStore).deleteJobVersions(0xc00028f7d0, 0x2711, 0xc0002e7680, 0xc000392100, 0xc0003a4040, 0x0)
	/go/src/github.com/hashicorp/nomad/nomad/state/state_store.go:1130 +0x1a1
github.com/hashicorp/nomad/nomad/state.(*StateStore).DeleteJobTxn(0xc00028f7d0, 0x2711, 0x175334f, 0x7, 0xc000306810, 0x2f, 0xc000392100, 0x0, 0x0)
	/go/src/github.com/hashicorp/nomad/nomad/state/state_store.go:1102 +0x46c
github.com/hashicorp/nomad/nomad/state.TestStateStore_DeleteJobTxn_BatchDeletes.func1(0xc000392100, 0x1777ce0, 0xc000392100)
	/go/src/github.com/hashicorp/nomad/nomad/state/state_store_test.go:1705 +0x1a2
github.com/hashicorp/nomad/nomad/state.(*StateStore).WithWriteTransaction(0xc00028f7d0, 0xc0000d5e48, 0x0, 0x0)
	/go/src/github.com/hashicorp/nomad/nomad/state/state_store.go:3953 +0x79
github.com/hashicorp/nomad/nomad/state.TestStateStore_DeleteJobTxn_BatchDeletes(0xc0001e2200)
	/go/src/github.com/hashicorp/nomad/nomad/state/state_store_test.go:1703 +0x685
testing.tRunner(0xc0001e2200, 0x1777138)
	/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:827 +0xbf
created by testing.(*T).Run
	/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:878 +0x353
```

burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,nak3/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,hashicorp/nomad,nak3/nomad