* This returns null if:
   *
   *  * The stream is exhausted.
   *  * We've read all of the bytes of an object whose length is known.
   *  * We've reached the [DerHeader.TAG_END_OF_CONTENTS] of an object whose length is unknown.
   */
  fun peekHeader(): DerHeader? {
    var result = peekedHeader

    if (result == null) {
      result = readHeader()
      peekedHeader = result
    }

    if (result.isEndOfData) return null