Fix for parsing a certificate with an extension attribute with a NULL-value

[mwpnl]

This pull request should resolve #37 by checking if a extension contains NULL data. When implemented, the extension is still given as output, but with an empty value (\0).

[yorkie]

By the way, please follow the coding style at first.

[Southern]

@mwpnl Should @yorkie or I make the changes that were suggested so that this can be merged in? Been a little over 2 weeks with no response.