Extensions->Render->Gear->Gear crash if Default document units = ft.

- Windows XP, Inkscape rev 13283.
- start with erased preferences file
- set Document Properties->Default Units = ft
- run Extensions->Render->Gear->Gear
- get Python crash message below:
- similar crash when running Render->Gear->Rack Gear
- similar crash when running Render->Random Tree
- similar crash when running Render->Wireframe Sphere
- not reproduced if 'm' or 'cm' are chosen
- not reproduced on Inkscape 0.48.4

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Traceback (most recent call last):

  File "render_gears.py", line 182, in <module>

    e.affect()

  File "C:\InkscapeBZR\inkscape\share\extensions\inkex.py", line 261, in affect

    self.effect()

  File "render_gears.py", line 88, in effect

    pitch_diametrial = float( teeth )/ pitch_diameter

ZeroDivisionError: float division