fix importing of certain stls in vismach

[sleepybishop]

i'm trying to integrate a 6dof robot arm with linuxcnc, been having problems with kinematics so i thought i'de try and model in vismach. once in a while an stl would fail to import, with the small changes here the failed stl imports went away.

submitting for merge consideration in case it's helpful, if not please disregard.

[andypugh]

Are you convinced that this is always better than the original version?
As far as I can tell the only sample config that uses an STL is VMC_Toolchange.

[sleepybishop]

yes i'm certain, before this code path would error out because [x1,x2,y1,y2,z1,z2] were not defined.

an example stl to trigger the problem can be found here:
https://github.com/sleepybishop/linuxcnc/blob/fishybearG-2.8/configs/sim/axis/vismach/ar2/ar2_link3.stl

[andypugh]

OK, if the original was actually buggy and crashy, then we should merge it.
Next question, though. If this is a bugfix then it should go in to 2.7 then be merged up to 2.8 and 2.9.
Can you possibly re-submit it against 2.7?
(if that's too much trouble then I can do it)