Get the current extractor properly.

This fixes the case where we need to switch from one variant to
another during preparation, having loaded zero samples, because
the first chunk for that variant gave us a 404 error. In this
case the first extractor will have no samples, and there will
be a second extractor that does.

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

@@ -130,7 +130,7 @@ public class HlsSampleSource implements SampleSource, SampleSourceReader, Loader
     }
     if (!extractors.isEmpty()) {
       // We're not prepared, but we might have loaded what we need.
-      HlsExtractorWrapper extractor = extractors.getFirst();
+      HlsExtractorWrapper extractor = getCurrentExtractor();
       if (extractor.isPrepared()) {
         trackCount = extractor.getTrackCount();
         trackEnabledStates = new boolean[trackCount];
@@ -221,7 +221,7 @@ public class HlsSampleSource implements SampleSource, SampleSourceReader, Loader
     Assertions.checkState(enabledTrackCount > 0);
     downstreamPositionUs = playbackPositionUs;
     if (!extractors.isEmpty()) {
-      discardSamplesForDisabledTracks(extractors.getFirst(), downstreamPositionUs);
+      discardSamplesForDisabledTracks(getCurrentExtractor(), downstreamPositionUs);
     }
     return loadingFinished || continueBufferingInternal();
   }