webm demo app: Fix crash on screen touch

Fix crash on screen touch before starting playback. mediaController cannot be
used until after setMediaPlayer has been called on it.

demo_misc/webm_sw_decoder/src/main/java/com/google/android/exoplayer/demo/webm/VideoPlayer.java

@@ -274,7 +274,7 @@ public class VideoPlayer extends Activity implements OnClickListener,
   }
 
   private void toggleControlsVisibility()  {
-    if (mediaController != null) {
+    if (mediaController != null && player != null) {
       if (mediaController.isShowing()) {
         mediaController.hide();
       } else {