Avoid asserting when painting certain degenerate bezier paths

It could happen that the stroker would attempt to join empty subpaths,
resulting in an invalid path structure that would cause assert later.

Task-number: QTBUG-43474
Change-Id: Ia369a31e60c40cdae3900d96f15f3e83c9e78b97
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>