Fix potential NPE if source is released without being prepared

Issue: #2264

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143550088

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

@@ -94,8 +94,10 @@ public final class HlsMediaSource implements MediaSource,
 
   @Override
   public void releaseSource() {
-    playlistTracker.release();
-    playlistTracker = null;
+    if (playlistTracker != null) {
+      playlistTracker.release();
+      playlistTracker = null;
+    }
     sourceListener = null;
   }