Ignore invalid position updates

Closes #2642

app/src/main/java/de/danoeh/antennapod/fragment/ExternalPlayerFragment.java

@@ -209,6 +209,10 @@ public class ExternalPlayerFragment extends Fragment {
     }
 
     private void onPositionObserverUpdate() {
+        if (controller.getPosition() == PlaybackService.INVALID_TIME
+                || controller.getDuration() == PlaybackService.INVALID_TIME) {
+            return;
+        }
         mProgressBar.setProgress((int)
                 ((double) controller.getPosition() / controller.getDuration() * 100));
     }