DndTile: Avoid NPE during boot phase

* Listening handling can only take place once mController
   is properly initialized.

Change-Id: Icbef2d506417f132e1ab47e23d457dacb9c00146

packages/SystemUI/src/com/android/systemui/qs/tiles/DndTile.java

@@ -235,6 +235,7 @@ public class DndTile extends QSTileImpl<BooleanState> {
     public void handleSetListening(boolean listening) {
         if (mListening == listening) return;
         mListening = listening;
+        if (mController == null) return;
         if (mListening) {
             mController.addCallback(mZenCallback);
             Prefs.registerListener(mContext, mPrefListener);