Fix NPE.

Backtrace looks like this:
E/AndroidRuntime(7061): java.lang.NullPointerException
E/AndroidRuntime(7061): 	at com.android.internal.util.cm.LockscreenTargetUtils.getDrawableFromResources(LockscreenTargetUtils.java:178)
E/AndroidRuntime(7061): 	at com.android.settings.cyanogenmod.LockscreenTargets.setTarget(LockscreenTargets.java:348)
E/AndroidRuntime(7061): 	at com.android.settings.cyanogenmod.LockscreenTargets.access$500(LockscreenTargets.java:55)
E/AndroidRuntime(7061): 	at com.android.settings.cyanogenmod.LockscreenTargets$2.onClick(LockscreenTargets.java:437)
E/AndroidRuntime(7061): 	at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
E/AndroidRuntime(7061): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(7061): 	at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(7061): 	at android.app.ActivityThread.main(ActivityThread.java:5289)
E/AndroidRuntime(7061): 	at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(7061): 	at java.lang.reflect.Method.invoke(Method.java:525)
E/AndroidRuntime(7061): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
E/AndroidRuntime(7061): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
E/AndroidRuntime(7061): 	at dalvik.system.NativeStart.main(Native Method)
W/ActivityManager(635):   Force finishing activity com.android.settings/.SubSettings

Change-Id: I72a5c7ff498ba0066f224a44985fadc64af4511c