Comment 14 for bug 781893

Comitted a workaround for the crash in revision 11547. It also adds some code to make the functionality more robust.

However, the issue is not solved yet. At some time a control point is created (maybe the/an invalid one) with the example file in comment 9. Depending on the workflow even more control points seem to be added. Afterwards while evaluating the snapping callback an invalid control point is encountered. One still has to prevent the creation of the invalid control point (invalid coordinates (nan) and desktop pointer (0xff)).
So Inkscape will still not respond well. If Inkscape shows weird behaviour (not continuing dragging), one has to click once on the canvas.