Issue when generating Grenze

Fontmake fails on the attached .glyphs file. Attempted with fontmake 1.8.0.

traceback
```
DEBUG:fontmake.font_project:ufoLib UFO validation is disabled
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.classes:Parsing "Grenze.glyphs" file into
DEBUG:fontmake.timer:Took 8.509s to run 'build_master_ufos'
INFO:fontmake.font_project:Building interpolation-compatible TTFs
INFO:ufo2ft:Pre-processing glyphs
Traceback (most recent call last):
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/bin/fontmake", line 11, in
sys.exit(main())
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/__main__.py", line 267, in main
project.run_from_glyphs(glyphs_path, **args)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/font_project.py", line 578, in run_from_glyphs
self.run_from_designspace(designspace_path, **kwargs)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/font_project.py", line 663, in run_from_designspace
**kwargs)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/font_project.py", line 723, in run_from_ufos
ufos, output_dir=master_bin_dir, **kwargs)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/font_project.py", line 250, in build_interpolatable_ttfs
self.save_otfs(ufos, ttf=True, interpolatable=True, **kwargs)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontTools/misc/loggingTools.py", line 375, in wrapper
return func(*args, **kwds)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontmake/font_project.py", line 425, in save_otfs
for font, ufo in zip(fonts, ufos):
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/ufo2ft/__init__.py", line 234, in compileInterpolatableTTFs
glyphSets = preProcessor.process()
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/ufo2ft/preProcessor.py", line 196, in process
remember_curve_type=self._rememberCurveType and self.inplace,
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/cu2qu/ufo.py", line 277, in fonts_to_quadratic
glyphs, cur_max_errors, reverse_direction, stats)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/cu2qu/ufo.py", line 152, in _glyphs_to_quadratic
segments_by_location = zip(*[_get_segments(g) for g in glyphs])
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/cu2qu/ufo.py", line 152, in
segments_by_location = zip(*[_get_segments(g) for g in glyphs])
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/cu2qu/ufo.py", line 101, in _get_segments
glyph.draw(pen)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/defcon/objects/glyph.py", line 471, in draw
self.drawPoints(pointPen)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/defcon/objects/glyph.py", line 478, in drawPoints
self._drawShallowLoadedContours(pointPen, self._shallowLoadedContours)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/defcon/objects/glyph.py", line 494, in _drawShallowLoadedContours
pointPen.endPath()
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontTools/pens/pointPen.py", line 141, in endPath
self._flushContour(segments)
File "/Users/marcfoley/Type/upstream_families/Grenze/venv/lib/python3.7/site-packages/fontTools/pens/pointPen.py", line 188, in _flushContour
assert len(points) == 1, "illegal line segment point count: %d" % len(points)
AssertionError: illegal line segment point count: 3
```

The problem may be related to the source file. However, It would be great if `--verbose DEBUG` logged which glyph it was currently pre-processing. This would help in debugging dodgy sources.

[Grenze.glyphs.zip](https://github.com/googlei18n/fontmake/files/2782565/Grenze.glyphs.zip)