Crash: ZeroDivisionError with dashed lines

[Smylers]

This HTML file crashes WeasyPrint (0.17) with a division-by-zero error: http://www.stripey.com/demo/weasyprint/dash.html

It works as intended if the border style is changed from dashed to dotted.

I can only get the crash to occur when the bordered element breaks across pages and contains a floated element, so possibly this is related to Issue #36.

Here's the stack trace:

Traceback (most recent call last):
  File "/usr/local/bin/weasyprint", line 9, in <module>
    load_entry_point('WeasyPrint==0.17', 'console_scripts', 'weasyprint')()
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/__main__.py", line 141, in main
    getattr(html, 'write_' + format_)(output, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/__init__.py", line 159, in write_pdf
    return self.render(stylesheets).write_pdf(target, zoom)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/document.py", line 393, in write_pdf
    page.paint(context, scale=scale)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/document.py", line 226, in paint
    draw_page(self._page_box, cairo_context, self._enable_hinting)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 61, in draw_page
    draw_stacking_context(context, stacking_context, enable_hinting)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 167, in draw_stacking_context
    draw_stacking_context(context, child_context, enable_hinting)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 137, in draw_stacking_context
    context, stacking_context.page, block, enable_hinting)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 67, in draw_box_background_and_border
    draw_border(context, box, enable_hinting)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 334, in draw_border
    side, border_edge, padding_edge)
  File "/usr/local/lib/python2.7/dist-packages/weasyprint/draw.py", line 477, in draw_border_segment
    dash = length / (2 * round(length / (2 * dash)))
ZeroDivisionError: float division by zero

[SimonSapin]

Fixed, thanks!