SystemUI: Add null check for wakelock in KeyguardViewMediator

* When SystemUI crashes, below crash appears making lockscreen blank.

Log:

04-11 10:42:00.060 24478 24478 E AndroidRuntime: FATAL EXCEPTION: main
04-11 10:42:00.060 24478 24478 E AndroidRuntime: Process: com.android.systemui, PID: 24478
04-11 10:42:00.060 24478 24478 E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.os.PowerManager$WakeLock.acquire()' on a null object reference
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at com.android.systemui.keyguard.KeyguardViewMediator.showLocked(go/retraceme c8e5bd84b42ae4e4ba71c46a73b244063826e864a9442c6ee58b8289e448cdd9:19)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at com.android.systemui.keyguard.KeyguardViewMediator.doKeyguardLocked(go/retraceme c8e5bd84b42ae4e4ba71c46a73b244063826e864a9442c6ee58b8289e448cdd9:195)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at com.android.systemui.keyguard.KeyguardViewMediator$12.handleMessage(go/retraceme c8e5bd84b42ae4e4ba71c46a73b244063826e864a9442c6ee58b8289e448cdd9:77)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:107)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at android.os.Looper.loopOnce(Looper.java:232)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:317)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:8498)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
04-11 10:42:00.060 24478 24478 E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:878

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>