Fix ChunkExtractorWrapper.init(TrackOutput) calls with null TrackOutput after extractor initialized

InitializationChunk calls init(null). When the initialization and index data is separate they need to
be loaded separately which results to two init(null) calls.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147837985

library/src/main/java/com/google/android/exoplayer2/source/chunk/ChunkExtractorWrapper.java

@@ -88,7 +88,7 @@ public final class ChunkExtractorWrapper implements ExtractorOutput, TrackOutput
       extractorInitialized = true;
     } else {
       extractor.seek(0, 0);
-      if (sampleFormat != null) {
+      if (sampleFormat != null && trackOutput != null) {
         trackOutput.format(sampleFormat);
       }
     }