Avoid exception when creating ChunkIndex with length = 0.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150908893

library/core/src/main/java/com/google/android/exoplayer2/extractor/ChunkIndex.java

@@ -61,7 +61,11 @@ public final class ChunkIndex implements SeekMap {
     this.durationsUs = durationsUs;
     this.durationsUs = durationsUs;
     this.timesUs = timesUs;
     this.timesUs = timesUs;
     length = sizes.length;
     length = sizes.length;
-    durationUs = durationsUs[length - 1] + timesUs[length - 1];
+    if (length > 0) {
+      durationUs = durationsUs[length - 1] + timesUs[length - 1];
+    } else {
+      durationUs = 0;
+    }
   }
   }
 
 
   /**
   /**