OGL crash with LineShape labels

Pierre Hjälm wrote:

> Here's a quick diff on lines.py (it's from 2.5.3.1, so I hope
> the line numbers are correct). Hopefully it won't break something else.
> The demo seems to work ok though.

Thanks Pierre, it seems to work for me also on another application using line labels (that was crashing before this patch).

Davide