Fix crash when open download

app/src/main/java/com/hippo/ehviewer/ui/scene/DownloadsScene.java

@@ -1155,7 +1155,9 @@ public class DownloadsScene extends BaseToolbarScene
                 return;
             }
             DownloadInfo info = mList.get(position);
-            holder.thumb.load(EhCacheKeyFactory.getThumbKey(info.gid), info.thumb);
+            if (info.thumb != null) {
+                holder.thumb.load(EhCacheKeyFactory.getThumbKey(info.gid), info.thumb);
+            }
             holder.title.setText(EhUtils.getSuitableTitle(info));
             holder.uploader.setText(info.uploader);
             holder.rating.setRating(info.rating);