Improve save wrong cover list
--> Do not save it at every request.
Avoid crash with NPE
ERROR/AndroidRuntime(27455): FATAL EXCEPTION: MPDAsyncWorker
        java.lang.NullPointerException
        at com.namelessdev.mpdroid.SearchActivity.asyncUpdate(SearchActivity.java:351)
        at com.namelessdev.mpdroid.SearchActivity$2.run(SearchActivity.java:260)
        at com.namelessdev.mpdroid.helpers.MPDAsyncHelper$MPDAsyncWorker.handleMessage(MPDAsyncHelper.java:291)

 ERROR/AndroidRuntime(27072): FATAL EXCEPTION: main
         java.lang.NullPointerException
         at com.namelessdev.mpdroid.fragments.NowPlayingSmallFragment.updateCover(NowPlayingSmallFragment.java:311)
         at com.namelessdev.mpdroid.fragments.AlbumsFragment.updateNowPlayingSmallFragment(AlbumsFragment.java:159)
         at com.namelessdev.mpdroid.fragments.AlbumsFragment.onMenuItemClick(AlbumsFragment.java:130)