Bad pointer in backups

[Aaron Bentley]

Tests of backups fail with panic: runtime error: invalid memory address or nil pointer dereference

As seen here:
http://reports.vapour.ws/releases/issue/589b59d9749a56085424ab0d

Example output
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x57d913]

goroutine 1514 [running]:
panic(0x1ef88c0, 0xc82000e0f0)
 /usr/lib/go/src/runtime/panic.go:464 +0x3e6
gopkg.in/mgo%2ev2.logf(0x26346a0, 0x3a, 0x0, 0x0, 0x0)
 /home/ubuntu/juju-core_2.2-alpha1/src/gopkg.in/mgo.v2/log.go:101 +0x93
gopkg.in/mgo%2ev2.(*mongoCluster).syncServersIteration(0xc820a3f400, 0x0)
 /home/ubuntu/juju-core_2.2-alpha1/src/gopkg.in/mgo.v2/cluster.go:550 +0x49a
gopkg.in/mgo%2ev2.(*mongoCluster).syncServersLoop(0xc820a3f400)
 /home/ubuntu/juju-core_2.2-alpha1/src/gopkg.in/mgo.v2/cluster.go:355 +0x23f
created by gopkg.in/mgo%2ev2.newCluster
 /home/ubuntu/juju-core_2.2-alpha1/src/gopkg.in/mgo.v2/cluster.go:78 +0x1d5
FAIL github.com/juju/juju/apiserver/backups 10.871s
=== RUN TestAll