Fix NPE releasing HlsMediaPeriod

Issue: #1907

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135798950

library/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaPeriod.java

@@ -103,10 +103,12 @@ import java.util.List;
   public void release() {
   public void release() {
     continueLoadingHandler.removeCallbacksAndMessages(null);
     continueLoadingHandler.removeCallbacksAndMessages(null);
     manifestFetcher.release();
     manifestFetcher.release();
+    if (sampleStreamWrappers != null) {
       for (HlsSampleStreamWrapper sampleStreamWrapper : sampleStreamWrappers) {
       for (HlsSampleStreamWrapper sampleStreamWrapper : sampleStreamWrappers) {
         sampleStreamWrapper.release();
         sampleStreamWrapper.release();
       }
       }
     }
     }
+  }
 
 
   @Override
   @Override
   public void prepare(Callback callback) {
   public void prepare(Callback callback) {