Celiagg backend freezes if passed a dash specification with total length 0 or less

[corranwebster]

The CPU goes to 100% and the app freezes.

This can be replicated by trying to draw a path with something like:

gc.set_line_dash([0.0, 0.0])

[corranwebster]

Simple fix is to add a sanity check in the set_line_dash method.