Verify source is not released before updating ad playback state

Updates to the ad playback state are posted on the main handler,
so they may arrive after the source has already been released
(=the internal MediaSource is null). This can cause NPEs.

PiperOrigin-RevId: 555102426

libraries/exoplayer_ima/src/main/java/androidx/media3/exoplayer/ima/ImaServerSideAdInsertionMediaSource.java

@@ -721,7 +721,9 @@ private void setContentTimeline(Timeline contentTimeline) {
 
   @MainThread
   private void invalidateServerSideAdInsertionAdPlaybackState() {
-    if (!adPlaybackState.equals(AdPlaybackState.NONE) && contentTimeline != null) {
+    if (!adPlaybackState.equals(AdPlaybackState.NONE)
+        && contentTimeline != null
+        && serverSideAdInsertionMediaSource != null) {
       Timeline contentTimeline = checkNotNull(this.contentTimeline);
       ImmutableMap<Object, AdPlaybackState> splitAdPlaybackStates;
       if (Objects.equals(streamRequest.getFormat(), StreamFormat.DASH)) {