Possible NullPointerException in ExoPlayerImplInternal.setRepeatModeInternal

When readingPeriodHolder and playingPeriodHolder are both null, a
NullPointerException is thrown when trying to reassign readingPeriodHolder.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=155635846

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

@@ -454,7 +454,7 @@ import java.io.IOException;
     if (!seenLoadingPeriodHolder) {
     if (!seenLoadingPeriodHolder) {
       loadingPeriodHolder = lastValidPeriodHolder;
       loadingPeriodHolder = lastValidPeriodHolder;
     }
     }
-    if (!seenReadingPeriodHolder) {
+    if (!seenReadingPeriodHolder && playingPeriodHolder != null) {
       // Renderers may have read from a period that's been removed. Seek back to the current
       // Renderers may have read from a period that's been removed. Seek back to the current
       // position of the playing period to make sure none of the removed period is played.
       // position of the playing period to make sure none of the removed period is played.
       int playingPeriodIndex = playingPeriodHolder.index;
       int playingPeriodIndex = playingPeriodHolder.index;