Fix assertion fail on seek.

Issue: #214

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

@@ -171,7 +171,7 @@ public class HlsSampleSource implements SampleSource, Loader.Callback {
     if (isPendingReset() || extractors.isEmpty()) {
       return false;
     }
-    boolean haveSamples = prepared && haveSamplesForEnabledTracks(extractors.getFirst());
+    boolean haveSamples = prepared && haveSamplesForEnabledTracks(getCurrentExtractor());
     if (!haveSamples) {
       maybeThrowLoadableException();
     }
@@ -194,13 +194,7 @@ public class HlsSampleSource implements SampleSource, Loader.Callback {
       return NOTHING_READ;
     }
 
-    TsExtractor extractor = extractors.getFirst();
+    TsExtractor extractor = getCurrentExtractor();
-    while (extractors.size() > 1 && !haveSamplesForEnabledTracks(extractor)) {
-      // We're finished reading from the extractor for all tracks, and so can discard it.
-      extractors.removeFirst().release();
-      extractor = extractors.getFirst();
-    }
-
     if (extractors.size() > 1) {
       // If there's more than one extractor, attempt to configure a seamless splice from the
       // current one to the next one.
@@ -318,7 +312,30 @@ public class HlsSampleSource implements SampleSource, Loader.Callback {
     maybeStartLoading();
   }
 
+  /**
+   * Gets the current extractor from which samples should be read.
+   * <p>
+   * Calling this method discards extractors without any samples from the front of the queue. The
+   * last extractor is retained even if it doesn't have any samples.
+   * <p>
+   * This method must not be called unless {@link #extractors} is non-empty.
+   *
+   * @return The current extractor from which samples should be read. Guaranteed to be non-null.
+   */
+  private TsExtractor getCurrentExtractor() {
+    TsExtractor extractor = extractors.getFirst();
+    while (extractors.size() > 1 && !haveSamplesForEnabledTracks(extractor)) {
+      // We're finished reading from the extractor for all tracks, and so can discard it.
+      extractors.removeFirst().release();
+      extractor = extractors.getFirst();
+    }
+    return extractor;
+  }
+
   private void discardSamplesForDisabledTracks(TsExtractor extractor, long timeUs) {
+    if (!extractor.isPrepared()) {
+      return;
+    }
     for (int i = 0; i < trackEnabledStates.length; i++) {
       if (!trackEnabledStates[i]) {
         extractor.discardUntil(i, timeUs);
@@ -327,6 +344,9 @@ public class HlsSampleSource implements SampleSource, Loader.Callback {
   }
 
   private boolean haveSamplesForEnabledTracks(TsExtractor extractor) {
+    if (!extractor.isPrepared()) {
+      return false;
+    }
     for (int i = 0; i < trackEnabledStates.length; i++) {
       if (trackEnabledStates[i] && extractor.hasSamples(i)) {
         return true;

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

@@ -205,6 +205,7 @@ public final class TsExtractor {
    *     for the specified track. False otherwise.
    */
   public boolean hasSamples(int track) {
+    Assertions.checkState(prepared);
     return sampleQueues.valueAt(track).peek() != null;
   }