[LinuxCNC/linuxcnc PR#370] Fixes #328. Preventing item assignment on tuple.

Trying to resolve #328, succeeded.
When one of rotary axis is wrapped, the code between line 1292 and 1302 will do item assignment in positions.
But if self.getshowrelative() returns false, positions is a tuple, item assignment will fail. So I added 2 lines of code to ensure positions is always a list.