Release DownloadHelper automatically if preparation failed.

This prevents further unexpected updates if the MediaSource happens to
finish its preparation at a later point.

Issue:#5915
PiperOrigin-RevId: 249439246

RELEASENOTES.md

@@ -11,6 +11,9 @@
   ([#5891](https://github.com/google/ExoPlayer/issues/5891)).
 * Add ProgressUpdateListener to PlayerControlView
   ([#5834](https://github.com/google/ExoPlayer/issues/5834)).
+* Prevent unexpected `DownloadHelper.Callback.onPrepared` callbacks after the
+  preparation of the `DownloadHelper` failed
+  ([#5915](https://github.com/google/ExoPlayer/issues/5915)).
 
 ### 2.10.1 ###
 

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadHelper.java

@@ -951,6 +951,7 @@ public final class DownloadHelper {
           downloadHelper.onMediaPrepared();
           return true;
         case DOWNLOAD_HELPER_CALLBACK_MESSAGE_FAILED:
+          release();
           downloadHelper.onMediaPreparationFailed((IOException) Util.castNonNull(msg.obj));
           return true;
         default: