RO: skip drawing arrow if start near end

[bertsky]

fixes a rare bug where reading order display could crash:

Traceback (most recent call last):
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/util/gtk.py", line 109, in _run
    callback()
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/util/gtk.py", line 66, in __call__
    self.callback(*self.args, **self.kwargs)
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/view/page.py", line 378, in redraw
    self.page_image, self.region_map = renderer.get_result()
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/model/page_xml_renderer.py", line 548, in get_result
    canvas, regions = self.operations.paint(self.canvas.copy())
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/model/page_xml_renderer.py", line 406, in paint
    operation.paint(draw, regions)
  File "/data/ocr-d/ocrd_all/venv/lib/python3.7/site-packages/ocrd_browser/model/page_xml_renderer.py", line 336, in paint
    lf = self.size / (d[0] ** 2 + d[1] ** 2) ** 0.5
ZeroDivisionError: float division by zero