Fix potential NPE in ExoPlayerImplInternal

I'll have a more thorough refactor of some of this class
fairly soon!

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139451693

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

@@ -918,7 +918,7 @@ import java.io.IOException;
       Object uid = loadingPeriodHolder.uid;
       int index = timeline.getIndexOfPeriod(uid);
       if (index == C.INDEX_UNSET) {
-        boolean restarted = attemptRestart(playingPeriodHolder.index, oldTimeline, timeline);
+        boolean restarted = attemptRestart(loadingPeriodHolder.index, oldTimeline, timeline);
         finishSourceInfoRefresh(manifest, true);
         if (!restarted) {
           // TODO: We should probably propagate an error here.