Really fix the NPE in ExoPlayer.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176821463

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

@@ -947,12 +947,14 @@ import java.io.IOException;
     MediaPeriodHolder periodHolder =
     MediaPeriodHolder periodHolder =
         playingPeriodHolder != null ? playingPeriodHolder : loadingPeriodHolder;
         playingPeriodHolder != null ? playingPeriodHolder : loadingPeriodHolder;
     while (periodHolder != null) {
     while (periodHolder != null) {
+      if (periodHolder.trackSelectorResult != null) {
         TrackSelection[] trackSelections = periodHolder.trackSelectorResult.selections.getAll();
         TrackSelection[] trackSelections = periodHolder.trackSelectorResult.selections.getAll();
         for (TrackSelection trackSelection : trackSelections) {
         for (TrackSelection trackSelection : trackSelections) {
           if (trackSelection != null) {
           if (trackSelection != null) {
             trackSelection.onPlaybackSpeed(playbackSpeed);
             trackSelection.onPlaybackSpeed(playbackSpeed);
           }
           }
         }
         }
+      }
       periodHolder = periodHolder.next;
       periodHolder = periodHolder.next;
     }
     }
   }
   }