Div by zero in AggMathStroke.StrokeCalcCap

[studylamp]

Had a case during some testing where StrokeCalcCap got called with Len = 0. This if course caused a division by zero crash on Da := 1 / Len.

As a hack fix, I just added a test to exit the procedure if Len is 0, but I am not sure what the implications of this are.

[CWBudde]

At the moment I don't have time to look into the issue, but maybe you can report back (or even send a PR) once you know whether this fix seems to work or not.