Guard against null TrackSelections in updateTrackSelectionPlaybackSpeed.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176629070

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -941,8 +941,10 @@ import java.io.IOException;
     while (periodHolder != null) {
     while (periodHolder != null) {
       TrackSelection[] trackSelections = periodHolder.trackSelectorResult.selections.getAll();
       TrackSelection[] trackSelections = periodHolder.trackSelectorResult.selections.getAll();
       for (TrackSelection trackSelection : trackSelections) {
       for (TrackSelection trackSelection : trackSelections) {
+        if (trackSelection != null) {
           trackSelection.onPlaybackSpeed(playbackSpeed);
           trackSelection.onPlaybackSpeed(playbackSpeed);
         }
         }
+      }
       periodHolder = periodHolder.next;
       periodHolder = periodHolder.next;
     }
     }
   }
   }