Prevent setting the non-indexed media artworkUrl that is not saved on disk

If the cache is cleared in the meantime, the app will only try to retrieve the
artworkUrl file that is not existing and then fail
Fixes #2600

application/vlc-android/src/org/videolan/vlc/util/ThumbnailsProvider.kt

@@ -95,8 +95,10 @@ object ThumbnailsProvider {
             BitmapCache.addBitmapToMemCache(getMediaCacheKey(true, media, width.toString()), bitmap)
             if (hasCache) {
                 media.setThumbnail(thumbPath)
-                if (media.id > 0) BitmapUtil.saveOnDisk(bitmap, thumbPath)
-                media.artworkURL = thumbPath
+                if (media.id > 0) {
+                    BitmapUtil.saveOnDisk(bitmap, thumbPath)
+                    media.artworkURL = thumbPath
+                }
             }
         } else if (media.id != 0L) {
             media.requestThumbnail(width, 0.4f)