Clear saved state from previous NURBSes to avoid bogus motion

Previously if an incorrect NURBS specification was given, such as when
there aren't enough control points, an error would be generated, but a
second load would succeed because there were leftover control points
from last time.  This caused weird behavior such as every second program
load succeeding (when it should have failed every time), and then of
course giving bogus motion when run.
NURBS: fix biarc G1 continuity

The center of the second half of each biarc was wrong.