github.com/juju/juju/api invalid memory address

[Curtis Hovey]

As seen in
    http://reports.vapour.ws/releases/issue/5829cc8b749a564d7479baa0

Mongo fell over

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x1086dfc0]

goroutine 6353 [running]:
panic(0x125b6800, 0xc82000c0e0)
 /usr/lib/go-1.6/src/runtime/panic.go:481 +0x3a0
gopkg.in/mgo%2ev2.logf(0x131c9780, 0x3a, 0x0, 0x0, 0x0)
 /home/ubuntu/juju-core_2.1-beta1/src/gopkg.in/mgo.v2/log.go:101 +0x90
gopkg.in/mgo%2ev2.(*mongoCluster).syncServersIteration(0xc82089d700, 0x0)
 /home/ubuntu/juju-core_2.1-beta1/src/gopkg.in/mgo.v2/cluster.go:550 +0x430
gopkg.in/mgo%2ev2.(*mongoCluster).syncServersLoop(0xc82089d700)
 /home/ubuntu/juju-core_2.1-beta1/src/gopkg.in/mgo.v2/cluster.go:355 +0x1f8
created by gopkg.in/mgo%2ev2.newCluster
 /home/ubuntu/juju-core_2.1-beta1/src/gopkg.in/mgo.v2/cluster.go:78 +0x1f4
FAIL github.com/juju/juju/api 85.770s