undertakerSuite.TestAPICalls nil deref panic

CI tests fail for master on i386 with a nil deref panic in the undertaker suite.

<http://reports.vapour.ws/releases/issue/567818f7749a566b61aaad25>

<http://reports.vapour.ws/releases/3448/job/run-unit-tests-trusty-i386/attempt/69>

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x1 pc=0x81dfa9b]

goroutine 17 [running]:
runtime.panic(0x8723a80, 0x9241cc8)
 /usr/lib/go/src/pkg/runtime/panic.c:266 +0xac
sync/atomic.StoreUint64(0x196121e4, 0x914f0000, 0x4e94, 0x807ce23)
 /usr/lib/go/src/pkg/sync/atomic/asm_386.s:196 +0xb
github.com/juju/juju/worker/undertaker_test.(*clock).advanceAfterNextNow(0x196121e0, 0x914f0000, 0x4e94)
 /home/ubuntu/juju-core_1.26-alpha4/src/github.com/juju/juju/worker/undertaker/undertaker_test.go:46 +0x45
github.com/juju/juju/worker/undertaker_test.func·001()
 /home/ubuntu/juju-core_1.26-alpha4/src/github.com/juju/juju/worker/undertaker/undertaker_test.go:84 +0x193
github.com/juju/juju/worker/undertaker_test.func·004()
 /home/ubuntu/juju-core_1.26-alpha4/src/github.com/juju/juju/worker/undertaker/undertaker_test.go:102 +0x2c7
created by github.com/juju/juju/worker/undertaker_test.(*undertakerSuite).TestAPICalls
 /home/ubuntu/juju-core_1.26-alpha4/src/github.com/juju/juju/worker/undertaker/undertaker_test.go:108 +0x32d
...
FAIL github.com/juju/juju/worker/undertaker 0.239s

Only commit since the previous run where this test passes is:

<https://github.com/juju/juju/commit/bb5ea3b064616f3a32fa25d8bb61eab3e07c9c7a>

Merge of pr #4002 from waigani/tidy-up-undertaker-test.