Don't query empty timeline for next/previous windows

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173668909

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

@@ -331,14 +331,14 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
 
   @Override
   @Override
   public int getNextWindowIndex() {
   public int getNextWindowIndex() {
-    return timeline.getNextWindowIndex(getCurrentWindowIndex(), getRepeatMode(),
-        getShuffleModeEnabled());
+    return timeline.isEmpty() ? C.INDEX_UNSET
+        : timeline.getNextWindowIndex(getCurrentWindowIndex(), repeatMode, shuffleModeEnabled);
   }
   }
 
 
   @Override
   @Override
   public int getPreviousWindowIndex() {
   public int getPreviousWindowIndex() {
-    return timeline.getPreviousWindowIndex(getCurrentWindowIndex(), getRepeatMode(),
-        getShuffleModeEnabled());
+    return timeline.isEmpty() ? C.INDEX_UNSET
+        : timeline.getPreviousWindowIndex(getCurrentWindowIndex(), repeatMode, shuffleModeEnabled);
   }
   }
 
 
   @Override
   @Override

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

@@ -564,7 +564,7 @@ public abstract class Timeline {
       boolean shuffleModeEnabled) {
       boolean shuffleModeEnabled) {
     switch (repeatMode) {
     switch (repeatMode) {
       case Player.REPEAT_MODE_OFF:
       case Player.REPEAT_MODE_OFF:
-        return windowIndex == getLastWindowIndex(shuffleModeEnabled)  ? C.INDEX_UNSET
+        return windowIndex == getLastWindowIndex(shuffleModeEnabled) ? C.INDEX_UNSET
             : windowIndex + 1;
             : windowIndex + 1;
       case Player.REPEAT_MODE_ONE:
       case Player.REPEAT_MODE_ONE:
         return windowIndex;
         return windowIndex;