Fix NullPointerException enabling WebVtt subtitles in DASH

Issue: #2596

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151013802

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DefaultDashChunkSource.java

@@ -179,15 +179,15 @@ public class DefaultDashChunkSource implements DashChunkSource {
 
     RepresentationHolder representationHolder =
         representationHolders[trackSelection.getSelectedIndex()];
-    Representation selectedRepresentation = representationHolder.representation;
-    DashSegmentIndex segmentIndex = representationHolder.segmentIndex;
 
+    if (representationHolder.extractorWrapper != null) {
+      Representation selectedRepresentation = representationHolder.representation;
       RangedUri pendingInitializationUri = null;
       RangedUri pendingIndexUri = null;
       if (representationHolder.extractorWrapper.getSampleFormats() == null) {
         pendingInitializationUri = selectedRepresentation.getInitializationUri();
       }
-    if (segmentIndex == null) {
+      if (representationHolder.segmentIndex == null) {
         pendingIndexUri = selectedRepresentation.getIndexUri();
       }
       if (pendingInitializationUri != null || pendingIndexUri != null) {
@@ -197,6 +197,7 @@ public class DefaultDashChunkSource implements DashChunkSource {
             trackSelection.getSelectionData(), pendingInitializationUri, pendingIndexUri);
         return;
       }
+    }
 
     long nowUs = getNowUnixTimeUs();
     int availableSegmentCount = representationHolder.getSegmentCount();