Handle empty PRIV frames

Issue: #2486

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148217936

library/src/main/java/com/google/android/exoplayer2/metadata/id3/Id3Decoder.java

@@ -404,6 +404,11 @@ public final class Id3Decoder implements MetadataDecoder {
 
   private static PrivFrame decodePrivFrame(ParsableByteArray id3Data, int frameSize)
       throws UnsupportedEncodingException {
+    if (frameSize == 0) {
+      // Frame is empty.
+      return new PrivFrame("", new byte[0]);
+    }
+
     byte[] data = new byte[frameSize];
     id3Data.readBytes(data, 0, frameSize);