panic when running restore -b

[Cheryl Jennings]

As seen in CI: http://reports.vapour.ws/releases/3836/job/functional-ha-backup-restore/attempt/3535

Running restore -b results in the following panic:

2016-03-30 17:07:59 INFO juju.cmd supercommand.go:37 running juju [1.25.5-precise-amd64 gc]
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x5eceb3]

goroutine 1 [running]:
runtime.panic(0x19ec9e0, 0x44dcb08)
 /usr/lib/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/juju/juju/cmd/juju/backups.(*RestoreCommand).rebootstrap(0xc21006d9c0, 0xc2101d5eb0, 0x55bbdf, 0xc2100a4900)
 /workspace/juju-core-1.25.5/src/github.com/juju/juju/cmd/juju/backups/restore.go:162 +0x33
github.com/juju/juju/cmd/juju/backups.(*RestoreCommand).Run(0xc21006d9c0, 0xc2101d5eb0, 0x12, 0x7f3a4c440c28)
 /workspace/juju-core-1.25.5/src/github.com/juju/juju/cmd/juju/backups/restore.go:203 +0x3e
github.com/juju/juju/cmd/envcmd.(*environCommandWrapper).Run(0xc2102bfc80, 0xc2101d5eb0, 0x7f3a4c5f0720, 0xc2102bfc80)
 /workspace/juju-core-1.25.5/src/github.com/juju/juju/cmd/envcmd/environmentcommand.go:1 +0x3d
github.com/juju/cmd.(*SuperCommand).Run(0xc2100875a0, 0xc2101d5eb0, 0x7f3a4c5f1380, 0xc2100875a0)
 /workspace/juju-core-1.25.5/src/github.com/juju/cmd/supercommand.go:427 +0x554
github.com/juju/cmd.(*SuperCommand).Run(0xc2100874b0, 0xc2101d5eb0, 0xc2101d5eb0, 0x0)
 /workspace/juju-core-1.25.5/src/github.com/juju/cmd/supercommand.go:427 +0x554
github.com/juju/cmd.Main(0x7f3a4c5f18a8, 0xc2100874b0, 0xc2101d5eb0, 0xc21000a010, 0x5, ...)
 /workspace/juju-core-1.25.5/src/github.com/juju/cmd/cmd.go:279 +0x283
github.com/juju/juju/cmd/juju/commands.Main(0xc21000a000, 0x6, 0x6)
 /workspace/juju-core-1.25.5/src/github.com/juju/juju/cmd/juju/commands/main.go:78 +0x460
main.main()
 /workspace/juju-core-1.25.5/src/github.com/juju/juju/cmd/juju/main.go:21 +0x44

Injected by: https://github.com/juju/juju/pull/4847

[Cheryl Jennings (cherylj)]

https://github.com/juju/juju/pull/4929