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

@@ -941,7 +941,9 @@ import java.io.IOException;
     while (periodHolder != null) {
       TrackSelection[] trackSelections = periodHolder.trackSelectorResult.selections.getAll();
       for (TrackSelection trackSelection : trackSelections) {
-        trackSelection.onPlaybackSpeed(playbackSpeed);
+        if (trackSelection != null) {
+          trackSelection.onPlaybackSpeed(playbackSpeed);
+        }
       }
       periodHolder = periodHolder.next;
     }