Correct an issue from pycrypodome switchover.

[mikepurvis]

Changes from #1609 broke reindexing decrypted bags:

Traceback (most recent call last):
  File ".../bin/rosbag", line 35, in <module>
    rosbag.rosbagmain()
  File ".../lib/python3/dist-packages/rosbag/rosbag_main.py", line 1012, in rosbagmain
    cmds[cmd](argv[2:])
  File ".../lib/python3/dist-packages/rosbag/rosbag_main.py", line 611, in reindex_cmd
    bag_op(args, True, True, lambda b: b.version > 102, op, options.output_dir, options.force, options.quiet)
  File ".../lib/python3/dist-packages/rosbag/rosbag_main.py", line 726, in bag_op
    outbag.close()
  File ".../lib/python3/dist-packages/rosbag/bag.py", line 715, in close
    self._stop_writing()
  File ".../lib/python3/dist-packages/rosbag/bag.py", line 1596, in _stop_writing
    self._write_connection_record(connection_info, True)
  File ".../lib/python3/dist-packages/rosbag/bag.py", line 1699, in _write_connection_record
    self._encryptor.write_encrypted_header(_write_header, self._output_file, header)
  File ".../lib/python3/dist-packages/rosbag/bag.py", line 309, in write_encrypted_header
    cipher = AES.new(self._symmetric_key, AES.MODE_CBC, iv)
  File ".../lib/python3/dist-packages/Cryptodome/Cipher/AES.py", line 232, in new
    return _create_cipher(sys.modules[__name__], key, mode, *args, **kwargs)
  File ".../lib/python3/dist-packages/Cryptodome/Cipher/__init__.py", line 79, in _create_cipher
    return modes[mode](factory, **kwargs)
  File ".../lib/python3/dist-packages/Cryptodome/Cipher/_mode_cbc.py", line 286, in _create_cbc_cipher
    if len(iv) != factory.block_size:
TypeError: object of type 'int' has no len()

I suspect this also impacted encryption security, since the iv parameter would have been too short by a factor of 8 and filled out with the deterministic padding.

Fix is confirmed to work with rosbag reindex on an encrypted bag under Python 2 and Python 3.

[dirk-thomas]

Thanks for the patch.