Event description is now correctly set and get from Outlook

Please @blaxter review as I'd like to include the unit tests you wrote for this to avoid regressions.

* Get event description from SOGo is not segfaulting
* RTF parsing is now working better for parsing control words and once we encounter NULL final char.
a \0 byte is a NULL terminated string. In my tests with Outlook, some spurious bytes were inserted at the end of the HTML due to having garbage once the RTF document is ended with \0 + GARBAGE. 

This will prevent that behaviour. In RTF specification, the \0 are not valid. So we have to end parsing. Do you have any other proposal?