Fixed NullpointerException in GpodnetAuthenticationActivity

src/de/danoeh/antennapod/activity/gpoddernet/GpodnetAuthenticationActivity.java

@@ -270,9 +270,11 @@ public class GpodnetAuthenticationActivity extends ActionBarActivity {
             @Override
             public void onClick(View v) {
                 final int position = spinnerDevices.getSelectedItemPosition();
+                if (position != AdapterView.INVALID_POSITION) {
                     selectedDevice = devices.get().get(position);
                     advance();
                 }
+            }
         });
     }