DndTile: Avoid yet another NPE during boot up phase

* Tile state update can only take place once mController
   is properly initialized.

Change-Id: Ia6c1d6dee7d3e396a5bb6910e03859a0c0cc94ae

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

@@ -174,6 +174,7 @@ public class DndTile extends QSTileImpl<BooleanState> {
 
     @Override
     protected void handleUpdateState(BooleanState state, Object arg) {
+        if (mController == null) return;
         final int zen = arg instanceof Integer ? (Integer) arg : mController.getZen();
         final boolean newValue = zen != ZEN_MODE_OFF;
         final boolean valueChanged = state.value != newValue;