private void setUndefinedState() {
    definedState = false;
    plaintextSegment.limit(0);
  }
Tries to read the header of the ciphertext.
@return true if the header has been fully read and false if not enough bytes were available
         from the ciphertext stream.
@throws IOException when an exception occurs while reading the ciphertextStream or when
        the header is too short.
java_src/src/main/java/com/google/crypto/tink/subtle/StreamingAeadDecryptingChannel.java