Bug report - NaN

[Ray Henry <rehenry-at-up.net>]

Fred et al

Ian sent me some flashcut nc code that he was running on EMC and said it
killed TkBackplot.  What it does is kill EMC. 

For my machine it caused a runaway that the soft limits don't see.  Only
metal on metal, or quitting emc, or an external e-stop power down, halts
this sucker.

---------------------------------------
G01 X 5.0000 Y 0.0000 
G02 X 5.0000 Y 0.0000 I-5.0000 J 0.0000         ########## Here Backplot
flips and takes off at high speed! #### 
---------------------------------------

My guess is that giving an I and J for an ending position that is the same
as the current position causes a divide by zero.  This happened in two
places in Ian's nc program.  I have no explanation why flashcut would allow
or use code like this but Ian is checking into it.

I presume that what we need to do is trap this problem in the interpreter.
(verify does not find it)  I think that we also need to add some kind of
orderly shutdown of the motion stuff somewhere in EMC whenever a NaN
occurrs.  

I'll be out for a few days.

Ray