[2.x] Clicking "+Z⇑" or "-Z⇓" throws a bogus error

[VanessaE]

Traceback (most recent call last):
  File "/home/vanessa/RepRap/Printrun-2.x/printrun/gui/zbuttons.py", line 149, in OnLeftDown
    if r >= 0:
TypeError: unorderable types: NoneType() >= int()

I refer to the extreme top and bottom of the Z axis jog control. Obviously the ends of the axis should do nothing since the actual Z controls are the three buttons below and above those spots. 😄

[hroncok]

Note that comparing different types was possible in legacy Python but it's not possible now.

[hroncok]

#910