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
(cherry picked from commit eb0e7aced70d16aec0f6b67c0bb5c290ab87be54)

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaServerSideAdInsertionMediaSource.java

@@ -704,7 +704,9 @@ public final class ImaServerSideAdInsertionMediaSource extends CompositeMediaSou
 
   @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)) {