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

[Alvin Penner]

- 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

...........................................

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

[su_v (suv-lp)]

Reproduced with r13283 on OS X 10.7.5 (Python 2.7.6)

[su_v (suv-lp)]

Not reproduced with 0.48.4 and 0.48.x r10025 on OS X 10.7.5 (Python 2.7.6).

[Alvin Penner (apenner)]

fix committed to rev 13288