Fix detection of TrueHD syncframe in MatroskaExtractor

MatroskaExtractor was checking for INDEX_UNSET as the sample count for buffers
not containing syncframes, but actually 0 was returned for these. Theoretically
this could prevent us starting to play a TrueHD stream as we wait until we can
read the sample count from a syncframe before accepting the audio in
DefaultAudioSink, but it seems that rechunking avoided this issue arising in
practice.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206575462

library/core/src/main/java/com/google/android/exoplayer2/extractor/mkv/MatroskaExtractor.java

@@ -1551,7 +1551,7 @@ public final class MatroskaExtractor implements Extractor {
       if (!foundSyncframe) {
       if (!foundSyncframe) {
         input.peekFully(syncframePrefix, 0, Ac3Util.TRUEHD_SYNCFRAME_PREFIX_LENGTH);
         input.peekFully(syncframePrefix, 0, Ac3Util.TRUEHD_SYNCFRAME_PREFIX_LENGTH);
         input.resetPeekPosition();
         input.resetPeekPosition();
-        if ((Ac3Util.parseTrueHdSyncframeAudioSampleCount(syncframePrefix) == C.INDEX_UNSET)) {
+        if ((Ac3Util.parseTrueHdSyncframeAudioSampleCount(syncframePrefix) == 0)) {
           return;
           return;
         }
         }
         foundSyncframe = true;
         foundSyncframe = true;

library/core/src/test/java/com/google/android/exoplayer2/audio/Ac3UtilTest.java

+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.audio;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.android.exoplayer2.util.Util;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+
+/** Unit tests for {@link Ac3Util}. */
+@RunWith(RobolectricTestRunner.class)
+public final class Ac3UtilTest {
+
+  private static final int TRUEHD_SYNCFRAME_SAMPLE_COUNT = 40;
+  private static final byte[] TRUEHD_SYNCFRAME_HEADER =
+      Util.getBytesFromHexString("C07504D8F8726FBA0097C00FB7520000");
+  private static final byte[] TRUEHD_NON_SYNCFRAME_HEADER =
+      Util.getBytesFromHexString("A025048860224E6F6DEDB6D5B6DBAFE6");
+
+  @Test
+  public void testParseTrueHdSyncframeAudioSampleCount_nonSyncframe() {
+    assertThat(Ac3Util.parseTrueHdSyncframeAudioSampleCount(TRUEHD_NON_SYNCFRAME_HEADER))
+        .isEqualTo(0);
+  }
+
+  @Test
+  public void testParseTrueHdSyncframeAudioSampleCount_syncframe() {
+    assertThat(Ac3Util.parseTrueHdSyncframeAudioSampleCount(TRUEHD_SYNCFRAME_HEADER))
+        .isEqualTo(TRUEHD_SYNCFRAME_SAMPLE_COUNT);
+  }
+}