Exposed by 16e4eeb9c0.
In (rare) occasions, a Curve's `CharInfo` can be NULL.
This also crashed going into editmode on affected text objects in
previous versions, however opening and rendering these files was still
fine.
In 4.0, opening those kind of files crashed.
It is not entirely clear how this can happen (there are comments in code
dating back to 97df61a7e5 mentioning "old file", there might be other
reasons, too), still, avoiding the crash on file open seems like good
practice.
Philipp Oeser