WifiTile: Avoid NPE during boot phase

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

Change-Id: Icd5003e67a725323a5445a936d5a38a3006c5431

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

@@ -76,6 +76,7 @@ public class WifiTile extends QSTileImpl<SignalState> {
 
     @Override
     public void handleSetListening(boolean listening) {
+        if (mController == null) return;
         if (listening) {
             mController.addCallback(mSignalCallback);
         } else {