Fixed minor error that occurred when the manifest was updated.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128472884

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

@@ -693,7 +693,7 @@ import java.util.ArrayList;
 
 
         Period previousPeriod = playingPeriod;
         Period previousPeriod = playingPeriod;
         boolean seenReadingPeriod = false;
         boolean seenReadingPeriod = false;
-        while (previousPeriod != null) {
+        while (previousPeriod.nextPeriod != null) {
           Period period = previousPeriod.nextPeriod;
           Period period = previousPeriod.nextPeriod;
           index++;
           index++;
           if (!period.id.equals(timeline.getPeriodId(index))) {
           if (!period.id.equals(timeline.getPeriodId(index))) {