crash - draw ellipse by center and three points

Here is how you can reproduce the crash:

- create a line
- turn on 'Snap on Endpoints'
- select the 'draw ellipse by center and three points' tool
- click on an endpoint of the line

...or just select the tool and click somewhere. ;p

fixed by https://github.com/LibreCAD/LibreCAD/commit/9c857e8b4b91d9ede2f9111c56250883b8293253

It's working. :D
Neat tool... thanks.