Android 2.1 - Fixed NullpointerException when request timeout while down...

Fixed NullpointerException on InputStreamReader when request timeout while downloading images.

This issue has occurred on Android 2.1 in Motorola Defy.

More details can be found in http://ivokee.com/log_error/show_all

Trace:
java.lang.NullPointerException at java.io.Reader.<init>(Reader.java:65) at java.io.InputStreamReader.<init>(InputStreamReader.java:65) at com.facebook.internal.ImageDownloader.download(ImageDownloader.java:248) at com.facebook.internal.ImageDownloader.access$1(ImageDownloader.java:205) at com.facebook.internal.ImageDownloader$DownloadImageWorkItem.run(ImageDownloader.java:356) at com.facebook.internal.WorkQueue$1.run(WorkQueue.java:118) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561) at java.lang.Thread.run(Thread.java:1096)

Thanks
Cristian Oliveira
It should still throw when the stream is null. When null, there should be a default error message: "Unexpected error while downloading image.".  That should be added to strings.xml with an name of com_facebook_image_download_unknown_error.  This should go to the else clause, and it should throw right after the if block.

Also, please prepare your PR against the dev branch instead of the master branch.