fb: fix possible NPE on power key handling

fixes reported NPE which can cause SoD

E/InputManager-JNI(  805): An exception was thrown by callback 'interceptKeyBeforeQueueing'.
E/InputManager-JNI(  805): java.lang.NullPointerException
E/InputManager-JNI(  805):      at com.android.internal.policy.impl.PhoneWindowManager.interceptKeyBeforeQueueing(PhoneWindowManager.java:4185)
E/InputManager-JNI(  805):      at com.android.server.wm.InputMonitor.interceptKeyBeforeQueueing(InputMonitor.java:357)
E/InputManager-JNI(  805):      at com.android.server.input.InputManagerService.interceptKeyBeforeQueueing(InputManagerService.java:1351)
E/InputManager-JNI(  805):      at dalvik.system.NativeStart.run(Native Method)
E/InputManager-JNI(  805): An exception was thrown by callback 'interceptKeyBeforeQueueing'.
E/InputManager-JNI(  805): java.lang.NullPointerException
E/InputManager-JNI(  805):      at com.android.internal.policy.impl.PhoneWindowManager.interceptKeyBeforeQueueing(PhoneWindowManager.java:4185)

Change-Id: Ia0aa32c5472d0381d629a83492b42dec419b840e