Avoid throwing an exception when an ID3 header is not found

Issue:#1966

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136836332

library/src/main/java/com/google/android/exoplayer2/extractor/ts/Id3Reader.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.extractor.ts;
 
+import android.util.Log;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.Format;
 import com.google.android.exoplayer2.extractor.ExtractorOutput;
@@ -28,6 +29,8 @@ import com.google.android.exoplayer2.util.ParsableByteArray;
  */
 /* package */ final class Id3Reader implements ElementaryStreamReader {
 
+  private static final String TAG = "Id3Reader";
+
   private static final int ID3_HEADER_SIZE = 10;
 
   private final ParsableByteArray id3Header;
@@ -82,7 +85,14 @@ import com.google.android.exoplayer2.util.ParsableByteArray;
           headerBytesAvailable);
       if (sampleBytesRead + headerBytesAvailable == ID3_HEADER_SIZE) {
         // We've finished reading the ID3 header. Extract the sample size.
-        id3Header.setPosition(6); // 'ID3' (3) + version (2) + flags (1)
+        id3Header.setPosition(0);
+        if ('I' != id3Header.readUnsignedByte() || 'D' != id3Header.readUnsignedByte()
+            || '3' != id3Header.readUnsignedByte()) {
+          Log.w(TAG, "Discarding invalid ID3 tag");
+          writingSample = false;
+          return;
+        }
+        id3Header.skipBytes(3); // version (2) + flags (1)
         sampleSize = ID3_HEADER_SIZE + id3Header.readSynchSafeInt();
       }
     }