Fix Tx3g decoding

Issue #1712

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129869936

library/src/main/java/com/google/android/exoplayer2/text/tx3g/Tx3gDecoder.java

@@ -18,6 +18,7 @@ package com.google.android.exoplayer2.text.tx3g;
 import com.google.android.exoplayer2.text.Cue;
 import com.google.android.exoplayer2.text.SimpleSubtitleDecoder;
 import com.google.android.exoplayer2.text.Subtitle;
+import com.google.android.exoplayer2.util.ParsableByteArray;
 
 /**
  * A {@link SimpleSubtitleDecoder} for tx3g.
@@ -26,13 +27,21 @@ import com.google.android.exoplayer2.text.Subtitle;
  */
 public final class Tx3gDecoder extends SimpleSubtitleDecoder {
 
+  private final ParsableByteArray parsableByteArray;
+
   public Tx3gDecoder() {
     super("Tx3gDecoder");
+    parsableByteArray = new ParsableByteArray();
   }
 
   @Override
   protected Subtitle decode(byte[] bytes, int length) {
-    String cueText = new String(bytes, 0, length);
+    parsableByteArray.reset(bytes, length);
+    int textLength = parsableByteArray.readUnsignedShort();
+    if (textLength == 0) {
+      return Tx3gSubtitle.EMPTY;
+    }
+    String cueText = parsableByteArray.readString(textLength);
     return new Tx3gSubtitle(new Cue(cueText));
   }
 

library/src/main/java/com/google/android/exoplayer2/text/tx3g/Tx3gSubtitle.java

@@ -26,12 +26,18 @@ import java.util.List;
  */
 /* package */ final class Tx3gSubtitle implements Subtitle {
 
+  public static final Tx3gSubtitle EMPTY = new Tx3gSubtitle();
+
   private final List<Cue> cues;
 
   public Tx3gSubtitle(Cue cue) {
     this.cues = Collections.singletonList(cue);
   }
 
+  private Tx3gSubtitle() {
+    this.cues = Collections.emptyList();
+  }
+
   @Override
   public int getNextEventTimeIndex(long timeUs) {
     return timeUs < 0 ? 0 : -1;