Fix check for last period index in ExoPlayerImplInternal

The if clause was never executed because nextLoadingPeriodIndex is set
to C.INDEX_UNSET instead of loadingPeriodIndex + 1.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159948661

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

@@ -1315,7 +1315,7 @@ import java.io.IOException;
           repeatMode);
     }
 
-    if (newLoadingPeriodIndex >= timeline.getPeriodCount()) {
+    if (newLoadingPeriodIndex == C.INDEX_UNSET) {
       // The next period is not available yet.
       mediaSource.maybeThrowSourceInfoRefreshError();
       return;