Fix edge case in HlsSampleSource.

1. prepare() needs to load a TsChunk to actually prepare the source.
2. Source is prepared, but no tracks are enabled (this is why it's
   an edge case - no-one is likely to be doing this!).
3. The TsChunk load completes.

We should not load additional chunks in this case.

library/src/main/java/com/google/android/exoplayer/hls/HlsSampleSource.java

@@ -345,7 +345,12 @@ public class HlsSampleSource implements SampleSource, Loader.Callback {
     if (!currentLoadableExceptionFatal) {
       clearCurrentLoadable();
     }
-    maybeStartLoading();
+    if (enabledTrackCount > 0) {
+      maybeStartLoading();
+    } else {
+      clearState();
+      allocator.trim(0);
+    }
   }
 
   @Override