Avoid message "Error while downloading image" upon synchronization when image caching is enabled.

Fix exception
> java.lang.IndexOutOfBoundsException
>   at java.util.LinkedList.remove(LinkedList.java:660)
>   at  de.luhmer.owncloudnewsreader.services.DownloadImagesService.StartNextDownloadInQueue(DownloadImagesService.java:148)
>   at de.luhmer.owncloudnewsreader.services.DownloadImagesService.onHandleIntent(DownloadImagesService.java:139)
>   at android.app.IntentService$ServiceHandler.handleMessage(IntentService.java:65)